Add GUI element (dropdown) to prepopulate format string with common complex formats

[bskinn]

Formats such as dates are long, and annoying to retype every time the app is loaded fresh.

While the durable settings of #10 would mitigate this somewhat, it would be convenient if the GUI helped out by providing some (hopefully) useful defaults.

[bskinn]

Would reduce problems/pain with situations like in #34.