proto: Add support for side mouse buttons
This is the couterpart of spice-protocol commit
commit cbe7b2c28543f4c5e57d1db1b753b73a64104162 (HEAD -> master, origin/master, origin/HEAD)
Author: SimonP <simonp.git@gmail.com>
protocol: Add support for side mouse buttons
Side mouse buttons currently do not exist in the protocol, causing them
to be inexplicably ignored by VMs in virt-manager and such. This lays
the groundwork for fixing that issue.
Signed-off-by: Frediano Ziglio freddy77@gmail.com