Shift Painting / Erasing should allow you to keep straight

[GlenDC]

In a similar way that other paint tools allow you to keep straight by holding shift, we should probably also be able to do this. This might change the way we implement painting and erasing. Right now they have an instant effect on a per-pixel base.

Testing other painting tools to see how they behave in the different situations regarding this feature should give a rough idea of what the user might expect from this feature.

This feature replaces the line tool described in issue #40.