Feedback for long running staging operations

### Is your feature request related to a problem? Please describe.
Working with a remote server where the connection is slow, staging can take a couple of seconds. After hitting space, nothing happens for a while. At some point, the staging is done, the staged file(s) become green.
During the waiting, you may be unsure of having hit your space bar correctly to initiate staging, especially when you have waited for several seconds already. So you hit space again. Now the waiting time becomes even longer and since you immediately unstaged, there is no feedback at all anymore, the file(s) just stay white.

### Describe the solution you'd like
During a staging process, there should be some feedback that you in fact initiated it. A spinner is used when pulling for example. The selected files may get a different selection color to indicate the hit. Lot's of possibilities to give feedback (though I assume consistency with the spinner would be the best)

### Describe alternatives you've considered
- Spinner
- Different background for selected files (requires a color per theme)
- Different text color for selected files (again, requires a color per theme)
- Floating "Staging..." window/widget

### Additional context
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