ui: Switch "-display sdl" to use the QAPI parser

The "-display sdl" option still uses a hand-crafted parser for its
parameters since we didn't want to drag an interface we considered
somewhat flawed into the QAPI schema. Since the flaws are gone now,
it's time to QAPIfy.

This introduces the new "DisplaySDL" QAPI struct that is used to hold
the parameters that are unique to the SDL display. The only specific
parameter is currently "grab-mod" that is used to specify the required
modifier keys to escape from the mouse grabbing mode.

Message-Id: <20220519155625.1414365-3-thuth@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>