gradle-ized project structure minor fix for tabcolors of none-extjs files

Restructured project layout to conform to gradle structure and added a small fix to FileTabColorProvider.getEditorTabColor to return null for none extjs files to let the ide use the default theme color.

To build the project a property needs to be passed to gradle to point to intellij installation, as in: 
gradle -PIDEA_HOME=path/to/intellij/homedir