Feature: on DnD of repo urls on window show Open Project dialog for url

Summary:
When browsing the web for some sources and finding the repo url,
this patch allows to drag and drop the url onto the KDevelop window
and automatically get the Open Project dialog opened, prefilled with
the matching vcs system and the url to use.

Future plans:

• extend for ProjectProvider plugins
• install protocol handlers and add app cmdline parameter -> KDevelop would be launched as handler for repo urls
• detect existing projects using a given repo url and ask if to open instead

Reviewers: KDevelop, mwolff

Reviewed By: KDevelop, mwolff

Subscribers: mwolff, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D4330