Add shortcut to expand/collapse notification panel

Use Ctrl+n to expand, Ctrl+Shift+n to collapse.

Fixes <https://github.com/Genymobile/scrcpy/issues/392>

README.md

@@ -267,6 +267,8 @@ you are interested, see [issue 14].
  | click on `VOLUME_DOWN`                 | `Ctrl`+`↓` _(down)_ (`Cmd`+`↓` on MacOS) |
  | click on `POWER`                       | `Ctrl`+`p`                    |
  | turn screen on                         | _Right-click²_                |
+ | expand notification panel              | `Ctrl`+`n`                    |
+ | collapse notification panel            | `Ctrl`+`Shift`+`n`            |
  | paste computer clipboard to device     | `Ctrl`+`v`                    |
  | enable/disable FPS counter (on stdout) | `Ctrl`+`i`                    |