Make Staged and Unstaged Changes to parallel tabs

**Is your feature request related to a problem? Please describe.**
Currently, `Staged Changes` and `Unstaged Changes` are separated blocks and take screen space simultaneously. This takes too much space, cannot show many changes in each, and we rare to read staged and unstaged simultaneously.

**Describe the solution you'd like**
make `Staged Changes` and `Unstaged Changes` to parallel tabs like `Files` and `Submodules` just like that in Tower. And indicate whether `Staged Changes` or `Unstaged Changes` is empty on the title.

**Describe alternatives you've considered**
At least, do not show (Un)staged Changes area when it's empty.

**Additional context**
Add any other context or screenshots about the feature request here.