This PR implements the first iteration of the preferences system.

The following features are working:

• Preferences are stored in a tree, thus nested preferences and categories are supported
• Support for booleans, reals and strings
• Definition of preference schema with default values and strong typing
• Loading and Saving of preferences from disk
• A rudimentary preferences dialog to view and change preferences

Defining preferences in the schema:
The preference tree schema is defined in PreferenceManager::InitializeDefaults(). The current placeholder properties can be replaced in later PRs.

The following features will be implemented in follow-up PRs, listed in order of implementation:

• Hidden preferences
• Specifying preference unit
• Color and font preferences
• Preference category separators to group preferences together in the dialog