Clicking on a destination that takes a while to mount should provide feedback while it loads

### Steps to Reproduce
1. Open Files
2. Click on a destination that usually takes a while to load (for me, this is an external, encrypted HDD)

### Expected Result
There should be some kind of feedback in the sidebar and/or content pane, such as a spinner or a message that the files are being loaded.

### Actual Result
It looks like nothing is happening and after five seconds or so the contents of the destination are suddenly there.

### Related
https://github.com/elementary/files/issues/132

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