cmake: Changed path we use to find gmock.

We now use the literal path that the archive decompresses to. Is there a way
to treat version numbers as variables when using cmake's find mech? For now,
we've hardcoded 1.7.0.
wm: Changed ctor test to default.

Wm will only ever have a default ctor, so the extra trappings are superfluous
atm.
wm: Added default ctor, empty monitors().
wm: add_monitor(), test for single add.
wm: Renamed test module.
wm: Added remove_monitor(), more single-monitor unit tests.
wm: Renamed test module to include window-manager.
wm: Added multiple-monitor add/remove unit tests.