Integrate QtVirtualKeyboard into KWin/Wayland

Summary:
The idea is to have KWin provide a virtual keyboard. To support this
KWin uses the QT_IM_MODULE qtvirtualkeyboard and makes sure that the
QPA plugin loads it.

KWin has a new class VirtualKeyboard which acts as the focus object and
the "proxy" for input methods. The QPA plugin ensures that this is the
focusObject, so that all input method related events are sent to this
class. From there it will be possible to delegate to other applications
through the Wayland interfaces.

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D1638