weston-keyboard: Enable to set as overlay panel

input-method-v1 protocol provides two surface type for the input
panel, `toplevel` and `overlay`. But there is no example for the later
one. This change enables to set weston-keyboard as overlay panel by the
environment variable `WESTON_KEYBOARD_SURFACE_TYPE=overlay` to
demonstrate this feature. In Addition, add weston.ini option
`overlay-keyboard` to set it.

Signed-off-by: Takuro Ashie <ashie@clear-code.com>