Fix Shift-Ctrl-arrows handling

Extending the selection with Ctrl-arrows is different from all the other
cases, as we need to combine both the selection anchor and the current
cell coordinates when doing it.

This means that we can't reuse the same PrepareForSelectionExpansion()
helper for this case, so this function is not useful finally and this
commit removes it entirely. It also replaces GetCurrentBlockCornerRow()
and GetCurrentBlockCornerCol() functions with GetExtensionAnchor() which
combines both of them.

Finally, it adds wxGridDirectionOperations::TryToAdvance() helper to
avoid repeating the IsAtBoundary() check which was previously part of
PrepareForSelectionExpansion() in multiple places.

And because the "extending" and normal parts of DoMoveCursorByBlock()
are so different now, it also factors out AdvanceByBlock() helper which
can be used to keep these parts well separate from each other instead of
intermixing them together.

With all these preparatory changes, it's finally possible to implement
the "extending selection by block" logic relatively easily, with the
bulk of this branch actually taken by comments explaining why do we have
to do what we do.

Add unit tests verifying that the functions used by Shift-Ctrl-arrow
work as expected.

include/wx/generic/gridsel.h

@@ -86,12 +86,12 @@ public:
                             const wxKeyboardState& kbd);
 
 
-    // Return the row of the current selection block if it exists and we can
-    // edit the block vertically. Otherwise return -1.
-    int GetCurrentBlockCornerRow() const;
-    // Return the column of the current selection block if it exists and we can
-    // edit the block horizontally. Otherwise return -1.
-    int GetCurrentBlockCornerCol() const;
+    // Return the coordinates of the cell from which the selection should
+    // continue to be extended. This is normally the opposite corner of the
+    // last selected block from the current cell coordinates.
+    //
+    // If there is no selection, just returns the current cell coordinates.
+    wxGridCellCoords GetExtensionAnchor() const;
 
     wxGridCellCoordsArray GetCellSelection() const;
     wxGridCellCoordsArray GetBlockSelectionTopLeft() const;