Implement "Preferences" Pane

[Matthew-McRaven]

We want a preferences editor heavily inspired by Qt Creator.

Users should be able to:

• [x] Adjust syntax highlighting styling
  • foreground color
  • background color
  • font options (e.g., strikethrough)
• [x] Adjust application theme
  • Application font color
  • Application font
  • Default background color
• [x] Support inheritance between options -- deferred for now, as we don't know if the effort is worth it for the number of "things" we have.
  • e.g., a comment inherits text and changes the background color to green
  • Inheritance loops must be explicitly rejected
• [x] Modify the application font
• [x] Handle common IO operations
  • Copy an existing theme
  • Save a theme to a file
  • Load a theme from a file
  • Prevent editing of builtin themes
• [x] Theme should apply to entire application (not just text editor) -- Finished in 8b5e591dd331b623eab9a98433caa6374dc88173

[Matthew-McRaven]

The following may be helpful so that the LHS and RHS can share a model: https://gist.github.com/Matthew-McRaven/ec7cd0f8b5ba46b2246ae78896898b7b