Optimization in KDE-PIM
Allow to share EntityTreeModel between several ETMCalendars.
This will be a big memory improvement for kontact.
Korganizer and kontact's summary view use different ETMCalendars
because they need different collection selections, this results
into all incidences being duplicated in memory.
Now it will be possible to have two ETMCalendars, with different
collection selections, using the same EntityTreeModel.