Implement "Open" command

Open username-removed-405114 requested to merge ffes/tortoisegit:opencommand into master

Fixes #2867

This MR Is the first try to implement the basics for a "Open" entry to open the webpage of the repo.

At the moment it is a basic implementation. It adds TortoiseGitProc /command:open that does work the the remote origin is on GitLab.com and GitHub.com. It is also possible to use the /remote:name argument although there is no UI for this at the moment.

It adds "Open" to the main context menu. For some reason I didn't manage to test this until now. The tips in src\Debug-Hints.txt don't seem to work for me, but I didn't want to hold back the MR.