on-screen keyboard: use style from plasma mobile

Right now, the keyboard looks like a desktop keyboard. Let's use the plamo-style, so it's mobile friendly.

@bshah wrote:

what plasma does is basically : https://invent.kde.org/plasma/plasma-phone-components/-/tree/master/qtvirtualkeyboardplugin

Copy style of the qt, and make some adjustments to it to be mobile friendly size

And then installs it to : QtQuick/VirtualKeyboard/Styles/ directory in qml install dir

And then finally sets env var : QT_VIRTUALKEYBOARD_STYLE=Plasma

Let's put that into a subpackage of plasma-phone-components, add it as dependency for postmarketos-ondev, and set the env var as well.