imwayland: Avoid TOGGLE_INPUT_PANEL requests if there's no focus

imwayland: Avoid TOGGLE_INPUT_PANEL requests if there's no focus

Fixes two things: 1) As GTK+ can be coerced into using the wayland IM
module despite the compositor not implementing the interface, all paths
not checking for global state before sending requests are prone to
crashes, this one fell hit this pitfall.

And 2) ensures the tap gesture only triggers TOGGLE_INPUT_PANEL if the
widget IM is focused. This is a possibility on eg. WebKit pages, where
its IM is only focused as long as a form element in the page is focused.
Tapping elsewhere shouldn't toggle the OSK.

https://gitlab.gnome.org/GNOME/gtk/issues/114

Closes: #114