Per-line stagging should be more user-friendly

[Escain]

Is your feature request related to a problem? Please describe.

The "+" little blue button to stage lines one-by-one is not really usable:

1. I could not figure how to add several lines at once: I would expect to press a starting line, move and release on the ending line. All the lines in the middle should be selected. Also, this should allow to scroll while the mouse is pressed.
2. When I click a line (on the little "+" blue button, it takes about 1.5s to update. It should (IMHO) be under 0.1s for such an operation that one would like to do 300times in a row.
3. When something is staged, it's not clear how to unstage it: I can select the "staged" version of the file and remove lines, but it should be IMHO more interactive: add/remove until I am ready.
4. Lines are staged both: at the left(remove) side and right(add) side at the same time. This relay on the fact that the diff match of lines is correct (which is usually not the case). I should be able to select diffs from A) left-only, B) right-only, or C) both.
5. The size of the "+" blue button is way too small.

In general, I recommend looking other similar applications (e.g. github-desktop) to see how this works and the feeling that I am looking for.

Describe the solution you'd like
Already explained above.

Describe alternatives you've considered
Already explained above.

Additional context
While staging long files.