[lipstick] Fix crash on close regression

6c6a924 reintroduced a segfault when closing lipstick. m_instance
was used after being reset to NULL.
[lipstick] Remove disconnectRef and listen availableWinIds changes

LipstickCompositor availableWinIdsChanged signal is connected
when a value of __winref window property is read for the first
time.
Merge pull request #212 from rainemak/master

[lipstick] Remove disconnectRef and keep listening availableWinIds
Merge pull request #218 from giucam/fix_crash

[lipstick] Fix crash on close regression