Add open-in-editor action

[ComradeVanti]

This PR implements functionality to open the current repository in an external editor. Below are the main changes:

1. Added setting for configuring external editor path
2. Add quick action for opening repo with editor
3. Add global menu action for opening repo with editor
4. Add CTRL+SHIFT+A shortcut for opening the repo

Things that could still be added:

• Validate that the entered path is actually a code editor
• Better error handling if no valid editor was specified. Currently, when I enter some random string like not an editor into the editor, it will still be saved and only fails once it tries to actually run that command. We then get an ugly error notification with some low-level error details. Would be nice to hide those
• Per-project overrides for the editor setting

A few notes:

• This is my first time contributing to this repo, so I was not sure what the best place to put things was. Please let me know if there are any corrections
• I broke the changes into a few small commits, which can be squashed if that is preferred

This PR partially resolves #230

[Escain]

Hi,
IMHO, this needs a bit more to be perfect:

1. Instead of adding an editor and override per repository, it would be better to use the OS default editor for a given file type.
   For example, I have a simple text-editor configured for .cpp and .hpp, but I use other programs for images, or markdown documentation.

2. This feature is IMHO intended for quick access to a single file, not to open the full project. That is, select a file of interest in a list and edit that file. Having a top-menu "Edit" that opens the full repository is in most of the case not even viable (several project, no evidence of what folder to look, what file to open, etc.)

With that, those 3 points of improvements (validate editor path, better handling of error and per-project override become useless).

In the hope it helps.

[ComradeVanti]

@Escain seems like you are talking about a separate feature. 'Open file' and 'Open project' are two different things. I understand you would like to see the 'Open file' feature added, but this has nothing to do with this PR, which is about 'Open project'. Maybe open a separate issue for your suggestion?

Having a top-menu "Edit" that opens the full repository is in most of the case not even viable (several project, no evidence of what folder to look, what file to open, etc.)

I don't know what you mean. GitHub Desktop has this feature and I use it regularly. I do it via shortcut instead of UI button, but still. If I have a project which is restricted to one folder and I would work on this project by opening that folder in say VS Code, then this is a common action to take at the start of a work session.

[Escain]

I see, so it just opens the folder with the given editor/explorer. Understood.