Skins should be allowed to have their own macros.vm files where to put global macros

Right now, the only possible thing is to define global macros in velocity.properties but only template macros.vm and a single skin's macros.vm can be specified, making it impossible to have more than 1 active skin with a macros.vm file. The solution is to have one Velocity Engine per skin (or rather per skin having a macros.vm file).