Capture and forward side mouse buttons (BTN_SIDE and BTN_EXTRA)
Adds support for capturing and forwarding the side mouse buttons some mouses have, typically used in web browsers for previous/next website in history. (Those are presented by GDK as buttons 8 and 9.)
spice-vdagent requires a corresponding patch to handle the buttons: spice/linux/vd_agent!40 (merged)
At the spice and protocol level, the required changes were previously merged:
This commit is a rebased and revised version of the patch by Kevin Pouget: https://www.spinics.net/lists/spice-devel/msg42582.html (I have confirmed with him that it's OK that I submit the updated patches.)