Getting the full list for a node can get quite expensive e.g. in
cases of recursive calls of QMakeProjectManager::findPriFile.

However, the FlatModel needs to decide quickly on whether an item
is editable to potentially allow renaming.

So split up QList<Actions> supportedActions() into individual
bool supportsAction(action) calls and make sure Rename is not
on the critical path.

Task-number: QTCREATORBUG-17953
Change-Id: I31841847f8aa7d7b94c63d76ce71efb1c930fa69
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>