Merge branch 'grid-selection-refactoring'

Completely overhauled selection handling in wxGrid.

Make various ways of extending selection (using Shift-arrow keys,
Ctrl-Shift-arrows, Shift-click etc) work as expected from the user point
of view instead of producing various bizarre results. Also improve
row/column header click selection as well as Ctrl/Shift-Space handling.

Internally, store selection as just a vector of blocks, independently of
the selection mode, and provide a simple API for iterating over it which
remains usable even with selections containing millions of cells (as
long as they're still composed of only a few blocks, which is the case
in practice).

Add more tests and add display of the current selection to the sample.

See https://github.com/wxWidgets/wxWidgets/pull/1772

include/wx/generic/private/grid.h

@@ -793,9 +793,37 @@ public:
     // boundary, i.e. is the first/last row/column
     virtual bool IsAtBoundary(const wxGridCellCoords& coords) const = 0;
 
+    // Check if the component of this point in our direction is
+    // valid, i.e. not -1
+    bool IsValid(const wxGridCellCoords& coords) const
+    {
+        return m_oper.Select(coords) != -1;
+    }
+
+    // Make the coordinates with the other component value of -1.
+    wxGridCellCoords MakeWholeLineCoords(const wxGridCellCoords& coords) const
+    {
+        return m_oper.MakeCoords(m_oper.Select(coords), -1);
+    }
+
     // 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)