listview: painting of bottom right rectangle was all wrong.

The little rectangle between the horizontal and vertical scrollbars was
painted all wrong.
listview: removed hard-coded control frame size values.

It now queries the style for the control frame size, and then calculates
the clip rectangle accordingly.
basegrid: removed hard-coded control frame size values.

The grid used to use hard-code control frame sizes. Now it queries the
current theme, and uses those returned values accordingly.
editcombo: removed BeginDraw/EndDraw calls which are not needed.

For a long time now those calls are not need. The base canvas class
already does all the double buffering for us.
editcombo: Removed hard-coded control frame sizes

Instead the widget now queries the current style for those values.
editcombo: Implements painting a ReadOnly state
editcombo: Publish the missing ReadOnly property