Resolve "Add support for custom keyboard shortcuts"

Preview

Todo

• Load user keymap user-keymap.json
• Use default accelerator if not set in user-keymap.json
• Save user keymap
• Apply user keymap on the fly
• Handle invalid accelerators
• Handle empty accelerators (disabled actions)
• ShortcutDialog should warn users about duplicate shortcuts
• Handle duplicate accelerators (prevent them)
• Pressing Enter in ShortcutDialog doesn't do the same as pressing "Apply"
• ShortcutEditor should group shortcuts, like in help overlay
• Create actions for everything that accelerators can trigger
  • Zoom In/Out
  • Reset Zoom
  • Focus tab 1..9
  • <complete list>
• Make use of actions across the entire app -- stop handling key combinations on a widget level
• Ensure Help Overlay does not contain hardcoded keybindings

Closes #143 (closed)