Themes improvements

Support for themes was added in #6566 and then merged in #6755. There were some ideas how to improve these in both pull requests. Saving them here and expanding them a little bit so those suggestions won't get lost and hopefully will be addressed someday.

- Modifying and creating new themes should be user friendly. Themes should not be defined in code - we shouldn't require people to know how to code to make xmage a little bit more pretty. Themes should be contained in a separate folder and defined by flexible, easy to edit config (.yml file?). Question: should default theme be also treated as such? We probably don't want to make it modifiable.

- Themes list should be loaded dynamically at the application startup. Adding new theme to choose in preferences panel should not require recompilation of the client module. Invalid theme should not stop client app from loading, it should display errors in the log file and not display faulty theme in preferences panel.

- Themes should be update compatible (update removes all default folders except few like plugins/images)

- Some colors, buttons and tints are not modifiable by themes right now. Perhaps sound package should also be a part of a theme.

- Themes should be able to inherit some values from the default theme (because maybe we don't want to override everything), so providing incomplete config (to some extent) would be still okay.