Use a separate desktop file for registering as kdev4 file handler

Summary:
The main desktop file is used by everyone who just wants to start kdevelop
e.g. from the menu. The exec line having -p as hard-coded parameter
will result in failed start, due to missing parameter behind -p when
invoked not as handler for a kdev4 file.
Separate registration as handler solves this.

Reviewers: KDevelop, apol, antonanikin

Reviewed By: KDevelop, apol

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D8867