Fix xdg-mime default registration

[jaimergp]

Description

Closes #226. Instead of using xdg-mime default, we edit mimeapps.list manually. If an application is already registered as default, we won't override it. We always populate the "Added associations" list with our desktop file so it's there in the "open with" menu. We also handle cleanups now.

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[jaimergp]

@mrclary could you check if this PR fixes your Linux issues? Thanks!

[mrclary]

@jaimergp, sorry for the late follow up. I've been sick all last week.
So a few observations with this branch:

• Using the test JSON file I can reproduce your results and see FileTypeAssociation in the available applications list
• Removing %f from the Linux platform command (i.e. import sys, pathlib as p; p.Path(r'__OUTPUT_FILE__').write_text(r'')) results in FileTypeAssociation not available in the applications list.
• Removing MimeType and glob_patterns does not affect the result
• Modifying the command in any way does not affect the result, as long as %f is somewhere in the command. e.g. "command": ["%f"] without MimeType or glob_patterns works

[jaimergp]

Perfect! I'm reading this as "it works as long as %f is present in some way", so I added a note to the docs. In that case, I think this is ready to go, right?

[mrclary]

Okay, I'm a bit embarrassed. I was not aware of the %f requirement, and Spyder appears in the applications list even for menuinst=2.1.1 if %f is included in the command. I apologize for the inconvenience; perhaps this PR was not necessary. Nevertheless, everything seems to be working as expected.

[jaimergp]

I did find a couple bugs and polished the cleanup, so no worries, it was indeed useful! Glad you made it work too.

[marcoesters]

I also think that this is PR is very useful: the documentation is clearer now and a subprocess call was replaced with a simple parser using only the standard library. To me, that's a win.

Just one small comment.