[Spreadsheet] Refactor keyboard handling

LineEdit no longer actually handles motion, it simply indicates which
action was taken to cause it to lose focus (e.g. which key was pressed).
It's up to the client code to determine what this means. This allows
significant consolidation of keyboard-handling logic, and the
implementation of more extensive keyboard navigation features.

New keyboard shortcuts include a tab counter to implement auto-return,
plus Ctrl->Arrow, End, Home, Ctrl-End, and Ctrl-Home, matching the
behavior of OpenOffice, LibreOffice, etc.

Block selection via keyboard has also been added by holding down the
shift key during navigation with the arrow keys (this also works in
combination with the Ctrl modifier for region navigation).

src/Mod/Spreadsheet/App/Sheet.cpp

@@ -1063,6 +1063,11 @@ bool Sheet::isMergedCell(CellAddress address) const
     return cells.isMergedCell(address);
 }
 
+App::CellAddress Spreadsheet::Sheet::getAnchor(App::CellAddress address) const
+{
+    return cells.getAnchor(address);
+}
+
 /**
  * @brief Set column with of column \a col to \a width-
  * @param col   Index of column.