Virtual Keyboard SNI: Improve text and usability

Summary:
This way we can see from the system tray its state.

CCBUG: 405397

Reviewers: #plasma, #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: ngraham, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D19898