[C++] Improve configuration for `get_config` command

This implements many improvements for the Configuration mentioned in !145 (comment 990427):

• Consistency fix for setDefault(key, def) by not marking the key as used
• Consistency fix for get(key, def) by using setDefault(key, def)
• Adds KVPGroup and KVPUsage enums for better filtering of all/user/internal keys and if they are used/unused
• Replace getAll with getKVPs using the above mentioned enums
• Replace merge with update function that only updates used values
• Moves assemble function to Dictionary class instead (will be used now in combination with getKVPs)