isMaster is not enough as long as we differ between master pointers and
keyboard. With flexible device classes, the usual checks for whether a
master device is a pointer (currently check for ->button, ->valuators or
->key) do not work as an SD may post an event through a master and mess this
check up.
Example, a device with valuators but no buttons would remove the button
class from the VCP and thus result in the
IsPointerDevice(inputInfo.pointer) == FALSE.
This will become worse in the future when new device classes are introduced
that aren't provided in the current system (e.g. a switch class).
This patch replaces isMaster with "type", one of SLAVE, MASTER_POINTER and
MASTER_KEYBOARD. All checks for dev->isMaster are replaced with an
IsMaster(dev).
Both values need to fail the request, so we need to check them before
looping through the modifier masks. Otherwise, a wrong grab_window will
show up in each failed modifier return but not actually cause an error.
