[Enhancement] Modular Structure of Theme Definition

[kevinmatthes]

As mentioned in #30 by @mcobzarenco, the colour themes shall be outsourced from the source code in future releases. With this issue, I would like to suggest a procedure on how the themes could be added very easily.

zee has a configuration directory in which the settings are stored. I would like to suggest to add a themes directory within this directory to store the themes in there. The themes themselves could be exported as RON (Rust Object Notation) files that can be added by the user. When the command to change the theme is triggered, a file selection dialogue just as for the file opening can be triggered to show the available themes and select them by name. The theme directory should be scanned for valid themes first when the selection dialogue is triggered.

Longer term, the theme directory should support subdirectories to clone Git repositories into which contain the zee themes. That way, all the current themes could saved in a new repository within this namespace as RON files and then be cloned into the configuration directory, at option.

I am looking forward for your feedback.