display current info

[romanbarth]

when looking at crops, it would be useful to somewhere display

• the filename (maybe even the complete path?)
• the current frame we're looking at to write in FrameStart/FrameEnd

[biswajitSM]

The filename already shows up in the mainbar of the program:

Now the frame numbers gets updated when you change and shows up in the line in the kymograph.