QWindowSystemInterface: add handleInputSourceAvailableChanged()

Add a new function to QWindowSystemInterface to inform whether
or not an input source is available. This information will
be used to determine if any items on screen should be drawn
with active focus.

Note that determining if an input source is available is not the
same as checking if the input panel is visible. On iOS the
input panel will hide when connecting an external usb/bluetooth
keyboard.

Change-Id: I860b7450b693a81b664b9cdc27fef64494358eb9
QWidgetWindow: clear focus on focus widget upon receiving QEvent::FocusOut

When the window looses focus (without being deactivated), clear focus
on the current focus widget. Do this to e.g stop cursors from blinking
inside line edits when the input panel is hidden.

Change-Id: I40a2314970b723b2c1b3c3fdf8715c8bfa50553c