Store a list of previously passed tiles

[dyoung522]

To speed up tests, we should only check tiles which have not previously passed validation (i.e. new or failed).

To this end, we should probably keep a database of tiles which have passed validation. Once a tile has passed, it's not likely to have changed.

Additionally, we should:

• store the tile creation date, and if it's changed, re-run the check (re-built tiles)
• provide an option to re-run all checks.

Possible ways to accomplish this:

• create a sqlite database to hold the data
• store a semaphore file in each tile directory

[dyoung522]

This currently works by creating an otv.db (sqlite database) file in the Ortho4XP directory. I don't like leaving files littered around, but since this util is system agnostic, I could not think of a better place to store it.