RFR CODETOOLS-7902256: jtreg plugin broken by 2018.2 changes

[Maurizio Cimadamore]

Hi,
this patch addresses some incompatible changes in the Intellij API 
brought by the newly released 2018.2 version. As usual, all the extra 
'overrides' do not use the @Override annotation, which would lead to 
failure when compiling against an older IntelliJ SDK. This approach 
served us well in the past and should guarantee compatibility with new 
and older SDKs.

I also added a bunch of changes related to improve the recovery when JDK 
project ant file DOM indexing fails. This is related to:

https://bugs.openjdk.java.net/browse/JDK-8208524

Which has been recently fixed.

P.S.
As usual, thanks to the Jetbrains folks (esp. Anna Kozlova) for 
providing guidance on how to implement the new methods :-)

Cheers
Maurizio