Scale decorations, icons, and buttons when hidpi is used

Enable automatic GTK scale handling. This allows decorators, window buttons, etc to be rendered in their proper size when window-scale = 1 or when window-scale = 2 (fractional scaling not testable yet here). Also render the window icon crisply in hidpi mode.

Note that it may be necessary to restart gtk-window-decorator after changing the scale factor due to not all functions calling it being re-run or updated when the scale factor changes