Shortcut serialization

[jpnurmi]

We need LogicalKeyboardKey <=> String conversion for (de-)serializing shortcut settings.

Flutter exposes some relevant bits and pieces but not a complete solution, so we're currently getting some help from GTK:

• https://docs.gtk.org/gtk3/func.accelerator_parse.html
• https://docs.gtk.org/gtk3/func.accelerator_name.html

Storing shortcuts formatted in the usual GTK way made more sense when we were using GSettings. Now that we have human-editable JSON config files, it would be nicer to have more human-friendly VSCode-like formatting for shortcuts.

GTK	VS Code
<Control><Shift>Down	Control+Shift+Down

Also, if we ever want to support other platforms then using GTK for the conversion might become a real problem. If Flutter doesn't offer a solution, is there any package out there that could help?