RFR 8196081: Add support for customized intellij project templates

[Maurizio Cimadamore]

Hi,
the webrev here:

http://cr.openjdk.java.net/~mcimadamore/8196081/

Contains a refresh of the IntelliJ project for JDK. Here's a list of the 
enhancements:

* Added support for overriding standard template settings with other 
ones defined in user-defined folder (this can be done by specifying the 
env variable TEMPLATES_OVERRIDES)

* Cleaned up unused part of the build.xml script used for 
building/cleaning/rebuilding the project; the new ant file is a thin 
wrapper around make, with no extra magic added; also removed unused ant 
variable from the ant.xml template file

* Fixed some broken settings: removed exclusion for src folder (which 
caused the VCS plugin not to highlight changes in the source code) and 
removed addition of test source root (as that causes red squiggly lines 
because some of the tests define mocked up versions of j.l.Object which 
confuse the IDE)

* Consolidate logic in idea.gmk by replacing ad-hoc target 
"FindIdeaModuleSrcDirs" with the official "FindModuleSrcDirs" one

Cheers
Maurizio