Checkpoint:  Refactoring Keyword Search Settings

Basically code completely, but still some issues that need to be fixed
before merging (#12, #13, #14 as examples).  Checking in a checkpoint
version so I don't go backwards trying to fix those issues.

Almostly completely refactored how keywords are set globally at the
search settings level and at each plugin/search level to address
fundamental flaws in the earlier design that prevented (well, made very
difficult and bug-prone) title-only matching on any plugin results.

Keyword setting sets and search keywords in the INI have not changed.
How they are used internally in the code has substantially.