Resizable: corner handles should be larger

[Scott González]

The corner handles, e.g., north-west handle, are square and therefore are either too small or extend into the content area.

See ​shedokan's image of how the handles should work.

[Jörn Zaefferer]

Something to consider for 2.0.0 rewrite. Though I doubt that its a good idea to make the handles even smaller. There's a reason why windows on OSX have a single, somewhat large handle and no single-pixel line handles.