ivi-shell: add input panel support

Input panels are used for complex text composition for CJK alphabets and for
onscreen keyboards. Support for this is already implemented in libweston and
the desktop shell.

This adds extends the IVI shell to add support for input panels as well. The
low-level parts are implemented in the IVI shell. The positioning of the input
panels is delegated to the controller.
Support for input panels and the relevant protocols is only enabled if the
controller attaches a listener to the new signals.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>