xwayland: add drag'n'drop support

• Add outgoing drag'n'drop support

• Fixes incr transfers (for big files)

• Refactor selection code

• Wayland → Xwayland

  • Create DND window
  • Send DND_ENTER
  • Send DND_POSITION
  • Send DND_DROP
  • Receive DND_STATUS
  • Send DND_LEAVE
  • Receive DND_FINISHED
  • Redesign wlr_data_source
  • Fix Firefox refusing text
  • Fix Chromium crashing in non-incr transfer: support multiple transfers at the same time
  • Only allow one transfer at a time
  • Cleanup wlr_xwm_selection_transfer when transfer is finished
  • Fix incr transfers
    • Big picture from epiphany to gimp
    • Big picture from chromium to lowriter
  • Make sure I didn't broke clipboard stuff

Will be done in another PR:

• Xwayland → Wayland
  • Figure out how the client knows about the Wayland window position: just draw a window that spans the whole screen
  • Figure out how to map X11 protocol to Wayland protocol
  • Receive DND_ENTER
  • Receive DND_POSITION
  • Receive DND_DROP
  • Send DND_STATUS
  • Receive DND_LEAVE
  • Send DND_FINISHED

Test clients:

• Firefox
• GTK+ (nice visual feedback)
• Chromium
• Gimp (has visual feedback too)

Fixes #295