Summary:
A little bit debug information about the current keyboard state is
useful. Thus a new tab is added to show information about xkbcommon.

It shows:
* layouts in the keymap
* currently active layout
* supported modifiers in key map
* currently active modifiers in state
* supported leds in key map
* currently active leds in state

Whenever a key is pressed/released the complete ui is updated to reflect
the latest state. That is pressing/releasing a modifier is directly
reflected in the ui.

This UI can extended as needed for further debug information about the
keymap state.

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3379