Agile Learner gives warning if you trade your only basic power/spell in a class for an advanced power/spell

[bminty]

When the user purchases Agile Learner for a class that only has a single basic power, Agile Learner will throw a warning when selecting that class to swap powers, that it is no longer valid.

As a separate, but similar issue, Agile Learner retains your class selection if you remove it, and then later add it back, even if the previous selection is no longer an option. For example I added agile learner, to Sourcerer (level 2), removed it, removed sorcerer entirely, added artisan level 2, added agile learner back, and the selection was still on sourcerer when it was added back even though it wasn't even on the sheet.

[kw]

Splitting off the issue of choice data persisting across purchases as #152.