Instead of loading the GtkBuilder .ui file we now create all widgets in
code.
Added several support functions to reduce code copy/pasting and making
additional widgets and supporting more metadata easier.
The overall layout should still look the same, with a few exceptions:
- Each notebook page only uses one grid. This makes it possible to align
all data entry widgets.
- Featured Organization and it's code were two treeviews next to each
other. These are now organized below each other to make automatic code
generation easier.
- Since we needed to touch this code anyway, I also fixed Xmp.dc.creator
and Xmp.iptcExt.ModelAge to be multiline. This closes #7286.
- The old icons used for the date button, add, and remove row buttons
were replaced by gimp-grid, list-add and list-remove.

Since this was the last .ui file used in GIMP plug-ins, we remove all
files from the .ui subdirectory and references to that.

Note that there are several more places where GtkBuilder is used, but
those cases uses strings defined in code instead of .ui files.