Add Markdown plugin

[igordsm]

This fixes #850 . Add a Markdown plugin with the following shortcuts:

• Ctrl+b for bold
• Ctrl+Shift+I for italic
• Ctrl+K for links

See a demo below. All three works with and without a selection (and place the cursor where it makes sense).

[cassidyjames]

Ah, yeah Ctrl+I is the jump-to-line feature. Hrm. @danrabbit thoughts on that?

[jeremypw]

See inline comments - review for functionality and regressions to follow.

[jeremypw]

I think all the shortcuts should work with <Control><Shift> otherwise its confusing whether or not you need to hold the shift key down.

[jeremypw]

The code works as expected and no regressions were found. Some coding issues are mentioned inline.

[igordsm]

@jeremypw Thank you for the reviews! I have addressed all but one (left a comment in the discussion above) and re-request a review.

I actually like using ctrl+shift more, since it feels more natural to apply the markups after selecting text.

Edit: I am not sure how I removed elementary/ux from the review. Sorry about that.

[jeremypw]

Works nicely now - thank! But you will need to add an entry to the next release in the appdata file. I'll reinstate the review request to the ux team.

[igordsm]

By the way, the Flatpak CI is returning with a very non-specific error (Failed to download sources: module gtksourceview: Failed to execute child process ?git? (No such file or directory)) that appears to be unrelated to the last commit. Is it supposed to work?

Edit: Nevermind. Works OK now.

[igordsm]

I have created a standalone repository https://github.com/igordsm/code-markdown-plugin that implements this PR and installs to the correct folders in Elementary 6.

I started a discussion on third party plugins at #908 . I would be happy to hear your @jeremypw @cassidyjames thoughts about that.

[danirabbit]

This seems to work as expected except with undo. It looks like for some reason undo history sees either 2 or 3 actions. For example:

1. Create some text foo
2. Highlight and Ctrl + Shift+ B for bold
3. Notice that the text is now **foo** as expected
4. Press Ctrl + Z for undo
5. Notice that the text is now **foo instead of foo

with links you'll notice that each bracket and the set of parentheses are treated individually

[davidmhewitt]

It looks like for some reason undo history sees either 2 or 3 actions

@igordsm If I remember correctly, the solution to this is to call TextBuffer.begin_user_action () at the start of an "action" (e.g. making something bold), then do all of the insertions and deletions necessary, then call TextBuffer.end_user_action (). The undo manager should then treat that as one action.

[igordsm]

@davidmhewitt Thank you. Just done that and it works as expected. @danrabbit undo/redo now work as expected.

[danirabbit]

Works as expected! Nice job