[1/2] Keyboard Layout applet

Convert KeyboardLayoutButton.qml from lookandfeel to a sharable workspace component, which can be reused for applet implementation.

• move KeyboardLayoutButton.qml to a shared components
• KeyboardLayoutButton.qml: expose layoutName{Short/Long} properties

https://bugs.kde.org/show_bug.cgi?id=390079

[2/2] Keyboard Layout applet

@davidedmundson