[SearchResultWidget] Small code cleanup
[FindInFiles] Add a new "Project" column in results

Addresses https://bugzilla.xamarin.com/show_bug.cgi?id=17426
[Core] Deprecate the 1:1 File->Project assumption

(As advised by Mike Krueger to complement the approach to
the fix for BXC#17426)
Merge pull request #557 from knocte/master

[FindInFiles] Add a new "Project" column in results