Add auto-install mechanism for local backends

[Ron89]

No one like a Long read before using a simple thesaurus query. So instead of posting where to find those blasted source files, it's better just download the thesaurus source on-demand when it is not available at indicated location.

[Ron89]

Break down the task into following steps:

• create a framework to load/set one time configurations
• create async mechanism for local backend file auto-downloading
• create config entry for prompting user to enable local backend auto-downloading.