qtwayland: Add support for 16 mouse buttons

Update expands the mouse button assignment code block, which formerly
handled only values 272, 273, and 274. New version handles all mouse
mouse buttons shown in kernel's input.h header (decimal values 272
through 285).
Value 286 appears to be the first button in the range for joystick
devices, so I did not include any other kernel-enumerated values.
(Other values might be possible- but I don't know of a mouse which
is currently being sold with more than 15 non-wheel buttons. The
mouse with 15 buttons is the Razer Naga, and I don't own it.)

Task-number: QTBUG-22759

Change-Id: I77f16eb89e24d7d6410199badff28c0d48f7df93
Reviewed-by: Jørgen Lind <jorgen.lind@nokia.com>