disable/adapt frame shifting when at the border

[kdarras]

related to #144 , it is weird that frame shifting works even when we have reached the start or end of the recording: pressing the left or right arrows will further compress the view.
We should disable the frame shifting when the start/end of the spectrogram is reached, and when we are near to the border such that a full frame shift cannot be made, we should only shift the frame as far as it is possible to move without changing the temporal extent (i.e., the zoomed-in box should keep the same width after shifting and thus only partly move towards the border).