Introduce graphical themes (default and dark)

What is a Theme?

A theme is a simple YAML file containing colors to use for the GUI.

Defaults themes will be located into the data/themes folder.
Additionnal themes can be created into $HOME/.nuxeo-drive/themes.

One can select a theme using the GUI option from the Settings or set the theme option into a configuration file.

Available Color Properties

This is the full list of customizable colors in those YAML files:

- background
    - accept_ok
    - conflict
    - error
    - progress_bar_1
    - progress_bar_2
    - shadow
    - text_selection
- text
    - error
    - icon
    - link
    - normal
    - placeholder

(attach screenshots with color names to ease understanding)

—

Ticket Scope

• Create the default, "light", theme into data/themes/default.yml.
• Create a "dark" theme, based on the dark mode on macOS, into data/themes/dark.yml.
• Set the existing and unused Options.theme option to "default".
• Add a theme selection list into the Settings.
• If a custom theme file is no more existant, display an error box and fallback on the default theme.
• If a custom theme file is not good (syntax error or missing color), display an error box and quit.