Allow opening level files in external text editor for viewing

[tobbi]

I often find myself wanting to look at the code of a level file. Unfortunately, our editor doesn't support that directly.

Thus, I've added this feature by abusing our "edit scripts" feature.

What still needs to be done

~~ - [ ] Add proper icon (currently uses save icon as a placeholder)~~

• Move to editor menu
• Maybe: Allow editing a level file in external editor and then refreshing the editor if the edited level file is valid, but I've talked to swagtoy on Discord and this seems to be difficult to accomplish.

Nonetheless, putting this up for discussion.