Add functions returning all definitions for a mimetype or file name

Summary:
This can be used to show UI controls to let the user decide which definition should be used
for a specific file.

Reviewers: vkrause, cullmann

Reviewed By: cullmann

Subscribers: cullmann, kwrite-devel, kde-frameworks-devel

Tags: Framework: Syntax Highlighting, Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D18997