various ui tweaks

[bragefuglseth]

Various changes to the UI to make it more elegant and robust. There are too many small ones to list here, so read through all of the changed code to see if there's anything you want to be different, but the most significant tweaks are:

• Making the image extend all the way to the top of the window in desktop mode
• Changing the "Dither It!" label into "Save Image" (I know, less fun, but it makes a lot more sense workflow-wise, as the dithered image is already visible)
• Moving the mentioned button into a bottom toolbar to avoid clutter
• Moving more properties into the breakpoint instead of setting them with bindings, to make things more tidy and concentrate adaptive properties in one place

Please let me know if you want anything changed!

[tfuxu]

Looks amazing!

Some small nitpicks:

• explanation popovers have inconsistent padding,
• F10 shortcut works even if sidebar is hidden,
• Custom headerbar CSS only works for close button,
• minor formatting inconsistencies

Nothing that I can't fix myself, after that both PRs should be ready for merge.

[tfuxu]

~Also, is visible-child-name: "desktop"; in line 77 needed?~ Nvm, it can be useful to indicate default headerbar state

[bragefuglseth]

The new changes look good to me.