Quick shortcuts to switch brushes

[dacap]

Add possibility to switch:

• brush type/angle/size,
• foreground/background color,
• ink type/opacity,
• shading colors,
• and/or pixel-perfect checkbox.

using Alt+1, Alt+2, etc. shortcuts.

I've create this issue from my comment here as this is a bigger issue than #787 (which will be kept open because there are some missing parts to be implemented yet)

Also, the implementation must be backward compatible with the current Edit > New Brush behavior.