Add mouse click

[jtroo]

This should be more easily cross platform than replicating the whole mouse.

Here's my use case: I have a trackball that is great for scrolling and movement but I don't like it much for clicking. So instead, when paired with my QMK board, I click with my left hand using the keyboard and navigate+scroll with my right hand using the trackball.

[jtroo]

Windows:
https://docs.microsoft.com/en-us/answers/questions/255509/send-mouse-wheel-click-properly-sendinput-mouseeve.html

[jtroo]

In Linux, these look promising. Haven't tested or confirmed on the internet though.

            272 => Some(OsCode::BTN_LEFT),
            273 => Some(OsCode::BTN_RIGHT),
            274 => Some(OsCode::BTN_MIDDLE),
            275 => Some(OsCode::BTN_SIDE),
            276 => Some(OsCode::BTN_EXTRA),
            277 => Some(OsCode::BTN_FORWARD),
            278 => Some(OsCode::BTN_BACK),

[jtroo]

Implemented in 76cebbf