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/private/grid.h

@@ -807,8 +807,23 @@ public:
     }
 
     // Increment the component of this point in our direction
+    //
+    // Note that this can't be called if IsAtBoundary() is true, use
+    // TryToAdvance() if this might be the case.
     virtual void Advance(wxGridCellCoords& coords) const = 0;
 
+    // Try to advance in our direction, return true if succeeded or false
+    // otherwise, i.e. if the coordinates are already at the grid boundary.
+    bool TryToAdvance(wxGridCellCoords& coords) const
+    {
+        if ( IsAtBoundary(coords) )
+            return false;
+
+        Advance(coords);
+
+        return true;
+    }
+
     // Find the line at the given distance, in pixels, away from this one
     // (this uses clipping, i.e. anything after the last line is counted as the
     // last one and anything before the first one as 0)