Add keyboard shortcut for default action

[Aethlas]

Description

Added the option to set keyboard shortcut through either text in options page (Firefox only) or through browser "manage keyboard shortcuts".

Shortcut changes in the browser will reflect the text box in options and vice-versa, but it recommended to set it through the browser since you have all those default keybinding checks included.
Only Firefox supports changing keyboard shortcuts dynamically (via commands.update() ), due "security concerns" you have to set it through chrome://extensions/shortcuts in Chrome. However, I kept the text box in, since it works on FF.

There should be no breaking change, but I have tested it on Firefox only.

Type of change

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• Code quality:
  • I have performed a self-review of my own code
  • I have commented my code, particularly in hard-to-understand areas
• Tests:
  • I have added tests that prove my fix is effective or that my feature works
  • New and existing unit tests pass locally with my changes
• Other:
  • I have made corresponding changes to the documentation and/or README.md
  • If new dependencies exist, I have checked that they are really necessary and agreed with the maintainers/community (we want to have as few dependencies as possible)