Improved PDF diagram export wizard user experience
* Page number addition is now an option in the wizard page
* The diagram selection tree no longer automatically expands itself
fully when any node is (un)checked. Automatic expansion is only done
up to the second node level only.
* The diagram filter now works with a 500ms quiet time delay which means
writing in the filter shouldn't slow down the UI like it used to.
* Initial selection is more properly taken into account when
initializing the wizard page. The containing model/index root is
always sought based on the selection and the diagrams contained by the
selected resource are then initially selected for that model in the
diagram list.
* The Select/Deselect Visible buttons have been removed and Expand and
Collapse buttons have been added instead for expanding and collapsing
the selected tree nodes.
* The Select All and Deselect All buttons now work differently. If any
filter is applied to the current tree view, only the diagrams shown in
the tree are selected/deselected when either button is pressed. If no
filter is applied, all diagrams will be selected/deselected.