Python Plugin Importer can now import directly from Web

(As per a discussion on krita-artists.)

Add a new action to the Tools -> Scripts menu: "Import Python Plugin from Web"

This lets you paste a URL from which to download and import the plugin. The URL can either be a link to a Github repository, in which case the plugin will automaticallly determine the zip download location, or it can be a direct link to a zip file on any website.

The existing plugin import action has been renamed to "Import Python Plugin from File"

Notes

1.) Regarding security with direct zip file downloads: this is circumventing browser warnings and virus checkers you might get via manual download; however, I'm unzipping the archives directly into memory and searching for python files there, so it should be impossible to trigger any malicious code in the process. (Unless the Python Plugin itself is malicious, but that won't be detected by virus checkers either)

2.) I wasn't quite sure how best to handle the "either enter URL or pick file from disk" in the UI without ending up with a long popup window chain. I think splitting it up into to menu entries is the best way to go, but I'm open for suggestions.