[Feature] Add visual feedback for lenghty computations

[niksirbi]

Is your feature request related to a problem? Please describe.
Some calculations on the napari widget (e.g. masking, alignment) take time, ranging from a few to tens of seconds.
Currently there is no indication that a calculation is taking place, which could lead to the mis-perception that the corresponding button click did nothing.

Describe the solution you'd like
Some sort of feedback (e.g. spinning hourgrlass, or progress bar) telling the user that the calculation is in process.

Describe alternatives you've considered
An "INFO" pop-up message acting as the feedback.