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This adds a bunch of snazz to the gltransitions demo. It is perhaps
a bit overloaded now, but it demos everything that we can do.

Changes:
 * The fire shader is now not a bin, it just renders an animating
   background with no textures involved.
 * The stacks don't all start on the same page.
 * The shaderbin passes the mouse coordinate to the shader.
 * The shaderbin allows specifying a "border" so that you can
   cause effects outside the bin child (something that is new to gtk4).
 * All the buttons and the stacks are now in shader-bins that runs
   a wobbly-widget effect based on the mouse position that
   wobbles outside the child allocation.

This gets around 500 instances at 60fps on my system.

Add adds a demo showing off GskGLShaderNode in various ways.

It has a transistion widget, using some examples from
gl-transitions.com, with child widgets being both images, a GL area
and real widgets (that let you edit the transition shaders
themselves.

It also has a fancy fire effect on hove on the buttons.

I was getting "assignment to varying fragColor" errors

GtkBuildable's get_name()/set_name() methods may shadow
GtkWidget's methods. Avoid that by renaming the API to
get_buildable_id()/set_buildable_id(), which also reflects
the name of the XML attribute the API refers to.

#3191

"Two of every kind..." Add some customized pickers
here. This found a number of issues that have been
fixed in the preceding commits.

Redo the tag insertion function to avoid quadratic
behavior, and at the same time, fix handling of
alpha for color attributes.

Update the copy of this function in gtk4-demo
as well.

Polish the text view demos a little bit.

Its almost a dictionary.

Show how to add a context menu to a custom widget,
and how to make a GtkPicture lookalike.

Make sure the callback functions in the .ui files are exported, so that
those functions can be recognized at runtime so that things run
properly.

Make the builder demo show up in searches for toolbar and
menubar.

Make the keyboard shortcuts work.

Make the toolbar look right.

Increase the inter-paragraph spacing in the info
text, to make paragraphs more clearly separated.

They should not both be active initially.

- make content prominent by using content-view style for the main grid.

We don't have any demos with a switch in headerbar.

Add an fps label to the scrolling demo.

Use tnum for the fps label to avoid jiggling.

Remove another copy of this code in the theme
switching benchmark.

When a group is selected (such as 'Benchmark'),
make the run button insensitive - there is nothing
to run.

Make gtk-demo show itself as a useful example.

Add the names of the main widgets as keywords to
our demos, but also things like "game". This helps
finding relevant demos in our growing list. You
can now for example type "label", and find the
"error states" and "links" demos showing GtkLabel
features.

Filter the sidebar on keywords that can be provided
by the demos. We extract keywords from the doc comment
at the top of each demo source by looking for words that
look like class names. We also allow to specify keywords
explicitly.

Ignore lines starting with # in the info comment at
the top of source files. This will be used to add
keywords for search in following commits.

This one is a more or less direct copy of the
settings dialog from widget-factory, demonstrating
error states and builder scopes.

In the video player demo, we have a button to make
the window fullscreen, but no easy way back. Add
the usual F11 keybinding, to make things at least
somewhat recoverable.

Add buttons to show the gtk-logo animation and
the Big Buck Bunny trailer.

Fix a corner case of input validation in the settings
dialog.

The animated background demos was referring to the
pixbuf demo that was removed a while ago. Reword
the text accordingly.

Add a simple demo for adding emblems to icons
using GdkPaintable.

Make it possible to have no background in
the animated paintable. This will be used
in a future demo.

Update all users.

The paintable demos were forgetting to clear
their window variable.

That is what it is.

Fixes: #2790