gtk3, gtk4: Adjust buttons

This MR continues !33 (merged), as well as addressing some other inconsistencies with QStyle buttons:

• Buttons
  • Button shadow now doesn't have any horizontal offset following Blue Ocean button changes
  • Shadow and background color is synced with QStyle now more or less
• Headerbar
  • Some apps (e.g. Lutris) require a bit bigger margins to have properly sized (not tall) buttons on headerbars, got them now
• Linked
  • There was one minor oversight so vertical linked items weren't working properly, fixed now
• Comboboxes
  • Non-linked comboboxes got shadows back
  • Combobox arrow appearance is now a bit more close to QStyle
  • Linked combobox (i.e. entry + button) now looks a bit closer QStyle ones

BUG: 443919

@teams/vdg