Support partial input mode

Support partial input mode for QWidgets, QML and QGraphicsViews.
Partial input mode refers to upcoming native feature, where virtual keyboard only covers lower part of the screen
(unlike the current default virtual keyboard that opens a specific fullscreen editing window).

This cannot be backported to earlier releases, as the native side (that the Qt utilizes) supports only
current version of Symbian release (aka Sym^3).

In native side, when splitview is activated, the system indicates to clients in API HandleResourceChange with a private
event that splitView is opening. At the same time, it reduces the ClientRect() of the application to be rect that is
visible above the keypad. AVKON application then resizes itself and most likely shows scrollbars to navigate the smaller application area.

We need to have something similar (at least ensure that the text edit widget is visible above the keypad) for Qt. For
Sym^3 we can do this without additional flags, if we declare the splitview functionality to be "default" (i.e. always active).
In later releases we can add flags to control the functionality (though I cannot see who would want to use the "old" way).