extending gui functionality

I've made some extensions to the gui:
- guiscale (through a GET variable 'guiscale') allows to change the size of video frames when displayed (useful when original video frames are tiny, as an alternative to upscaling video frames which effectively makes  loading hits slower)
- clipping (through a GET variable 'clipping') controls whether boxes are clipped (by default) {true,1} or not {false,0}
- clipping (through a ctrl key) if it's being held when a box is moved and released over the image border, clipping won't happen
- square boxes (using shift)
- center + extent boxes (through ctrl key) draws bb using first click as a center and second click to define the extent
- shift and ctrl can be used together