Treat main run configuration file as root

[sizmailov]

This PR uses main files from run configurations as primary definition of "root documents". This has advantages over current scheme:

• can be easily controlled by user
• better following of latex logic on complicated cases
• generally speaking a number of "root document contexts" comes in play. It becomes important when a file is included in multiple root document with different definitions, etc.

The current scheme is not bad, but it's collection of guessing after all. I think it should be used as fallback strategy (e.g. when there is no run configurations)

This PR is only first step in this direction. Actions/Intentions should be adjusted, "root document contexts" should be introduced.

Fixes #753

Additions

Changes

Changed PsiFile.isRoot check to be true for all main files in run configurations

Backwards incompatible changes

Probably they exist, but I'm not aware of any

[sizmailov]

I've tested failing test manually, I have not I idea what went wrong.

[PHPirates]

@sizmailov It appeared to have something to do with that the tests don't set up the index correctly. But I'm not going to fix that just for this, it sounds too complicated :)