Fixed size cropping and minor improvements / bugfixes

- Main new feature: in cropgui.py, you can select the size of the cropped image from menues among several standard sizes. This feature is partially implemented in cropgui_common.py, so it should be easy to add it to cropgtk.py. [dea4c6a, 3c8ac4c, b349397, 3409320, 22d06c2, 370f4fb]
- Internal representation of w,h,left,right,top,bottom changed to image coordinates. This prevents size changes when moving the crop region which occured due to rounding errors. [ef5ba8f, 1d1fb3e]
- Computation of scale by same algorithm in cropgui.py and cropgtk.py. The old algorithm in cropgui.py only gave powers of two. The old algorithm in cropgtk was completely different and could result in too small images (because it fit the larger image dimension in the smaller screen dimension, e.g. image.w into screen.h) [2babc8d]
- Misc small bugfixes [d53aedf, 236c151, 2714048]