TextEdit minimap should be themeable.

[catpieyum]

I've noticed that the TextEdit minimap's colors are hard-coded into the class itself and because of that it can't be themed in the same way GraphEdit's minimap can. It takes colors from a theme that is on the node but that is even more confusing in my opinion because it's not mentioned anywhere that this is the case unless you look at the code to see what exactly it does when it comes to colors.

TextEdit minimap code for reference.