Re-use logic from DUChainUtils in DeclarationContext

It had its own way of finding the item under the cursor, which behaved
slightly different in a corner case occuring in the python plugin.
Re-using the existing functions instead of reimplementing the
item finding solves the problem.
Session chooser: Fix "Create New Session" item

Make it possible to activate that item even if a filter has been set.
Session chooser: Fix selection of default entry

Before that patch, the default index was selected more or less randomly.
We cannot rely on the row number for remembering the row with the
default session, because the model is sorted afterwards.
Minor: Fix apidocs a bit
Merge remote-tracking branch 'origin/1.6'
Minor: Fix some strings