Add "legacy HiDPI" toggle and implement scaling mockups

This makes it look like this, if scale-monitor-framebuffer is enabled. Note! This does NOT toggle said setting; it relies on the corresponding functionality being enabled by something else. That means downstream patching, manual editing the gsetting, or at some point when it's decided that this should be enabled by default.

The "legacy HiDPI" toggle makes GNOME Shell switch between the "new" and "old" way without manipulating any experimental gsettings. This does not need any mutter changes, as it has been part of the D-Bus API for some years already.

Disclaimer: I'm not very well experienced in building GUIs, so please be gentle. The text preview thing is implemented using GtkScrollableWindow and an enlarged GtkInscription. I went with the Pango sample text instead of the Lorem Ipsum stuff - it is translated thus more relevant, especially for non-Latin-like languages.

It looks like this:

Mockups can be found here: #2516.

Some open questions I didn't find answers to in the mockups:

• There is no easy way to tell what scale active. The bold percent is for the default/preferred scale. The one with the blue frame is for the currently selected/pending scale.
• The mockups limits the selectable scales to 5. When the preferred/default is e.g. 250%, that means the current implementation shows 2 below, and 2 above, meaning the lowest is 200%. Not sure this is the best way to handle it.