Fix disabling blend mode combo for MyPaint brushes

MyPaint brushes don't support custom blend modes, they always use what
MyPaint calls "Normal and Erase". The code already made an attempt to
disable the blend mode combo box to represent that, but switching tools
caused it to re-enable itself, despite having no effect.

This commit fixes that: the brush blend mode combo box gets disabled
upon switching tools if a MyPaint brush preset is selected. It also adds
a comment explaining the reason behind that rather non-obvious special
case.

BUG: 445206