Dialog blocking

1) This adds a blocking status dialog to `Git Pull` and `Git add`.
2) The dialog is shown at least for 2 seconds to avoid flashy appearance.
3) Dialog needs to be made pretty.

Fixes #2360, #1567 

@dinhviethoa PTAL thanks
I think you should reset the progress message and progress indicator visibility.
```
_setProgressMessage("");
_toggleProgressVisible(false);
```

Could you extend the delay only if the git pull operation took less than 2 secs?