Reference selection overhaul:

• Set the active sub-region and element properly.
• Reactivate the reference resizing with the mouse.
  
  Selection:
• Warn, if a location can't be inserted.
• Reset sub-region to the whole region on ending the reference selection mode.
• more verbose debug output
• API docs.
• Remove unused parameter.
• Merge Selection::setActiveElement() into Selection::setActiveSubRegion.
• Clearing the selection in normal mode leaves the range A1 in.
• Add tests for Selection.
• Fix the anchoring after setting the active element/sub-region.
  
  CellEditor:
• Rearrange the updating of the active sub-region.
• Point beyond the last selection range, if the text cursor is not in a sub-region.
• More verbose debug output.
• Merge checkChoice() into slotTextChanged().
• Enable/disable the reference selection dependent on the content.
• Do no do the highlighting, if the reference mode got disabled.
• Do not suspend the reference mode on entering text.
• Move the rehighlighting to slotTextChanged(). Block the signals to prevent an infinite recursion.
• For an unknown reason, the external editor's text gets now deleted when the in-cell editor gets deleted. Fetch the user input before and pass it through.
• Seems non-sense to disable the reference mode, if the in-cell editor doesn't have focus. The external one may.
• Update the active range/sub-region on external editor's cursor changes.
• Remove superfluous member variable.
• Inherit KTextEdit instead of aggregating it.
• Rewrite the resizing on text changes.
• Disable the automatic appending of a percent char, if the style is set to format the value as percentage.
• Replace the eventFilter() by separate event handlers.
• Remove dead code.
• Move the enabling/disabling of the reference selection mode to slotTextChanged().
• Do not trigger function auto-completion, if not in reference mode.
• Replace sub-regions.
• Fix the editor focussing.
• Fix the cursor position interaction between the embedded and the external editor.
• Remove dead code.
• Paint the references with the correct color.
• Fix the formula expression coloring for ranges not on the origin sheet.
• The syntax highlighting gets automatically updated after text changes.
• Do not colorize invalid ranges.
• Do not trigger the function auto-completion, if the editor has no focus (is hidden).