Add button to dynamically resize virtual keyboard

Summary:
One of the things I dislike most about our virtual keyboard integration
is it's size. It's at least on my system covering too much vertical
space. Unfortuantely the keyboard API does not offer a way to control
the vertical space - one can only control the width. Thus it's also not
possible to just provide the keyboard in the optimal size.

This change tries to address the problem by adding a resize button. When
pressed one can dynamically decrease and increase the width of the
keyboard and thus also the height.

The button is added to the top/right corner. Ideally it would be added
to the symbol button row, but either I couldn't find the appropriate API
hooks or it's not possible.

Test Plan: Started KWin with the change and tested with touch input

Reviewers: KWin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D16485