* Update files in src/plugins

Change-Id: Ia5d77fad7d19d4bb3498e78661982f68729adb22
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>

Finding the class definition for a forward declaration or finding the
function definition from its declaration is mostly determined by the
file iteration order. Documents with the most common path prefix are
checked first.

This works fine as long as the files of your project have a common
ancestor. If that's not the case, FollowSymbol might take you to the
definition within another project.

Fix that issue by considering the project part id when constructing the
file iteration order. Since the cached file iteration order now depends
on the projects, ensure to clear it if projects are added, changed or
removed.

Task-number: QTCREATORBUG-15116
Change-Id: I529166bac363959c9fee0b946747fd0370a88809
Reviewed-by: Marco Bubke <marco.bubke@theqtcompany.com>