The BrushTool is very cool, but it doesn't allow us to re-use any
of the code in other classes. Many of the other tools have duplicated
code for handling mouse events / keeping track of previous location,
etc.
This commit sets up BrushTool so that other tools can inherit from
it and override some virtual functions to allow similar behavior
without re-writing the code to keep track of mouse positions, etc.
In particular, we add public setters/getters for `size` and
`hardness` properties, and make `draw_point()` and `draw_line()`
virtual so that derived classes can override them.
Note: We still pass in `color` as a parameter for `draw_line()` and
`draw_point()` instead of using `color_for()` directly because it
doesn't really make sense to be constantly asking the ImageEditor
for the color when it's not really changing (for instance along all
the points of a line)
fbfa6543ab