TexRenderer: abstract guidelines from drawing code

[ForNeVeR]

That's a follow-up for #71. I've decided to abstract the guidelines code from the Draw method, and implement the Template method pattern (with DrawWithGuidelines being the template method and Draw being the primitive) for the following purposes:

• keep Draw clean from the guidelines code
• regain principial compatibility with [WIP] Gotta draw 'em all on Geometry! #69
• remove the need to call base.Draw in Box-derived classes

Now every box uses DrawBackground method and every box supports guidelines (my previous implementation worked only for HorizontalBox and VerticalBox).

[alexreg]

Looks fine to me. Let's change the start-up project to the example project though, okay?

[ForNeVeR]

Yep, sure. Thanks for the review!