Removed check for pending events during mouse drag of GRC blocks on FlowGraph canvas.
This is always true on Windows with latest PyGTK and so blocks would never move with mouse movement.
Disabling the check appears to have no adverse effects.
Added PaintDC to 'plotter_base' so WX GL sinks will work under Windows.
Changed 'notebook' SetSelection (deprecated) to ChangeSelection.
Merge remote-tracking branch 'balint/windows_fixes' into maint