#2361 add support for linked style.mm

[Enrico Schnepel]

I have several mm files which are very similar. I am using custom node styles with automatic formating based on a rule set. Every time I create a new mm file I am using a style template file. But every time the style/rules change I have to update every mm file with the new styles/rule set.

Please add support for a linked style/rule mm file. In this case the formatting rules are not persisted in the linking mm-file it self but in the referenced style/rule file. The styles/rules in the style/rule mm-file are applied to every file which references to the style/rule file.

This might also a solution for feature request #1315.