Add the ability to erase partial masks via brush strokes and hotkeys (UX improvement)

[Alchete]

Currently, Gradio only supports mask re-painting via a single undo mechanism that progressively removes brush strokes.

In Photoshop masks are toggleable via the 'x' key, between black and white, and can be quickly painted and unpainted without one's hand leaving the mouse.

Potential implementation:

• Add 'x' key as a hot toggle between black & white when in mask mode
• Add a button next to the mask to toggle between black & white
• Change (or add) to the sequential "undo" mechanism with a "clear mask" button