Fix a bug where QuickDialogTableView would sometimes try to scroll to an indexPath beyond its bounds.

Removed QuickDialogTable::indexForElement: method because:

- it was only used in 2 places to obtain the NSIndexPath for the QElement to scroll to.
- it didn't take hidden QElements into account, which cause to NSRangeExceptions crashing an app

Instead, those 2 places now use QElement::getIndexPath which does take hidden QElements into account.
Use QElement::getIndexPath instead of QuickDialogTableView::visibleIndexForElement:

QuickDialogTableView::visibleIndexForElement: returns an incorrect
result after rows have been deleted dynamically from the tableview.

Since this method gives incorrect results and is only used in 1 place,
I deleted it and replaced it with QElement::getIndexPath which returns
the correct result.
Merge branch 'bug/fix_cellForElement_after_deleting_or_inserting_rows' into features/working_prevnext_buttons

Conflicts:
	quickdialog/QuickDialogTableView.m
When previous or next button tries to scroll to an offscreen cell, get the index path of the offscreen cell and not the currently selected cell.

This fixes a mistake made during merging.
Ensure that QEntryElements are deselected when pressing next or previous button.

This solves a bug where a QEntryElement with the keepSelected property set to YES would appear selected (selectedBackgroundView drawn) when recycled.
Merge pull request #442 from bartvandendriessche/features/working_prevnext_buttons

Fixes for previous / next buttons