Fix registration of kdevelop as handler of kdev4 files

Summary:
Also fix data of mimetype registration of kdev4 files:

• use application/ due to being an app-specific format
• set subtype
• set generic icon to kdevelop

Reviewers: KDevelop, apol

Reviewed By: KDevelop, apol

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D8853