Add point draw methods for styles 'dot' and 'dot-line'

[wimrijnders]

• Added point drawing routines for dot and dot-line
• Moved selection of point drawing method to separate method
• DRY'd color selection for graph styles bar and dot

Style dot-line was actually absurdly easy to implement; it's just dot
with an extra line.

[yotamberk]

I know we said this before, but please make sure all the examples are working. These are breaking changes and I usually sample some of the examples but I don't get to check all of them in-depth. I trust you know what you are doing =)
(We need to start considering a testing tool. As the features stack up, it's getting harder to test manually...)

[wimrijnders]

I test every time I make a pull request. Usually, I try out all the options in the tooltips demo to see if they all still work. For the current work, I think this is proof enough that it works.

As the features stack up, it's getting harder to test manually..

The thing to note is that with all these changes I'm making, the external working of Graph3d doesn't change at all! It's just one big refactoring 😉 .