Allow pan/resize for selected frame

[starturtle]

Right now, the selected frame must be drawn as a new rectangle each time. Instead, it should become a set of four edges that can be moved "independently".
As there is no real independence, there are several ways to cope:

• only allow dragging corners
• distribute equally among the adjacent edges (e.g. when moving the left edge of a square 10px to the left, move top edge up by 5 and bottom edge down by 5).
• distribute accordingly to the corresponding position (e.g. when moving the left edge of a square 10px to the left at 20% from the bottom, move up the top edge by 8px and down the bottom edge by 2px).
• "the winner takes it all": if less than 40% from the top, only move up the top edge by 10px, if less than 40% from the bottom, move down the bottom edge by 10px, otherwise distribute equally.

This should be possible to avoid tedious fixing of small errors when cutting the desired piece of image.