gtk: Fix releasing grab when closing popup

ptyxis-grab-again

We may be processing a GDK_BUTTON_RELEASE event without any known pressed keys if releasing after a wl_pointer.enter which unsets all modifiers.

• Tap with two fingers to open a context menu
• Quickly release both fingers, then hold one finger down
  • libinput will process this as holding down the right click
• Hover an action that would close the popup
• With the same finger, push down the touchpad to click
  • this will trigger a
    • release right-click (invokes the action, closes the popup)
    • press left-click (takes a grab on the focused widget)
    • wl_pointer::leave (leaving the popup surface)
    • wl_pointer::enter (entering the main window again, resets modifiers)
    • release right-click

We need to make sure we process this final GDK_BUTTON_RELEASE event to release the grab we took on the previous GDK_BUTTON_PRESS.

The same is not necessary in x11 because the prior GDK_BUTTON_PRESS was eaten by check_autoclose() instead, so no grab was taken.