Adding serialized save file and storing theme settings.

[toadkarter]

Resolves #8

CHANGES

• A new configuration file is now available for saving certain user specific-settings.
• If one does not already exist, the configuration file will be created at the following directory:

Linux and MacOS: if defined, $XDG_CONFIG_HOME/logicrs/config.json. If not, $HOME/.config/logicrs/config.json
Windows: %USERPROFILE%\AppData\Local\logicrs\config.json

• Currently, the settings file is only used for saving the theme, so that this persists between application sessions.
• If for some reason writing or reading from or to the settings file becomes impossible, an error message will be logged and the application will proceed under the assumption that user settings cannot be accessed.

TESTING CRITERIA

1. Open the application.
2. Change the theme by selecting an option from Settings -> Theme.
3. Close the application.
4. Restart the application and notice that the chosen theme from Step 2 is now loaded immediately.
5. Also notice that when accessing Settings -> Theme, the radio button will now be set to the currently selected theme.

Please do let me know if you have any thoughts on the implementation - I am very new to Rust so if you have any suggestions for improvements I would be very happy to implement them!

[Spydr06]

Awesome, excellent job!