optimize: implement hash-based lookup for itemsForUrl

When loading big projects with lots of files, much time was spent in
Project::itemsForUrl (up to a minute in total in one of my tests).

Now we use a QHash<uint, ProjectBaseItem*> lookup table, which brings
down the total time to a mere fraction of a second.

The lookup table is saved in the ProjectModel and is updated on demand
from the ProjectBaseItem.

Introduces a new ProjectModel::itemsForUrl which will return all files
for a url without filtering by project.