Introduce "InputMethod" singleton and supporting code to detect virtual keyboard

This adds an InputMethod singleton object that can report the type and
state of the current input method. It also includes some support code in
libkirigami to wrap the KWin VirtualKeyboard interface and uses that to
detect whether a virtual keyboard is currently being used.

While virtual keyboard is the only thing currently supported the intent
is that the InputMethod class can report on more types of input.
3 jobs for !398 with work/ahiemstra/inputmethod in 2 minutes and 42 seconds (queued for 6 seconds)
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passed android_qt515 #141846
Linux

00:01:54

passed freebsd_qt515 #141847
FreeBSD

00:00:41

passed suse_tumbleweed_qt515 #141845
Linux

00:00:43