input: Add support for zwp_text_input_v3 and zwp_input_method_v2

Some clients such as gtk+3 require the zwp_text_input_v3 protocol for
input-method handling. This patch adds this protocol support to text-backend and also
modifies weston-keyboard to support zwp_input_method_v2, which is an update
for the v1 protocol and aligned with the updated text-input protocol.

The patch also adds support for zwp_virtual_keyboard_v1, used to emulate the
behaviour of a physical keyboard.

Note that the updated input-method v2 protocol does not support the input-panel interface
as it did on v1. The input panel interface is implemented by desktop-shell to properly
display the virtual keyboard interface.
To avoid depending on both input method v2 and v1 (for input-panel only), this patch adds
a new weston-input-panel internal protocol (a copy of the old interface from input-method v1),
which is then implemented by desktop-shell and used by weston-keyboard when displaying
its window.

Signed-off-by: Andre Moreira Magalhaes (andrunko) <andre.magalhaes@collabora.co.uk>