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'
Merge branch 'master' of git://anongit.kde.org/kdevplatform into dark_theme_colors
Add text elide and don't resize value column