GTK 4, libadwaita, new HIG

[GeopJr]

Hashbrown should switch to GTK 4, use libadwaita and comply with the new HIG.

Currently crystal-gobject supports only GTK 3. However there's a promising lib for gtk4 available but as the readme mentions, it's not ready yet.

There were some attempts to use libhandy (some public some local) but since GTK 4 is here, it might be better to postpone them until there's stable support (for both gtk4 and libadwaita) in Crystal.

In regards of the new HIG, the plan is to:

• Move the from GtkStackSwitcher to AdwViewSwitcher (+ AdwViewSwitcherTitle & AdwViewSwitcherBar) with Title and Bar toggling dynamically based on window size.
• Move to AdwApplicationWindow
• Wrap labels and other widgets to support a wider variety of window sizes
• Use GtkListBox + rows instead of grids & boxes
• GtkPopoverMenu

(More items might be added to the above list)

[genodeftest]

Hashbrown should switch to GTK 4, use libadwaita and comply with the new HIG.

There is an issue with gtk4 which renders it pretty hard to use for people with non-HiDPI monitors which may or may not be worth waiting for a solution before publishing a gtk4 version of Hashbrown.

[GeopJr]

I'm sure that issue will be fixed by the time a gtk4 (+ libadwaita) library for Crystal becomes ready, thanks however for bringing it to my attention!