Allow no language to be selected for pptext

[cpeel]

Request
Change the pptext validation to allow no language to be selected as long as spellcheck is also not selected. Bonus: allowing pptext to dynamically load the list of available dictionary languages and populate the checkboxes.

Discussion
The following discussion was pulled from #9

@srjfoo:

[U]nder "Select wordlist language(s), is the implication that pptext should not be run if a text is in a language not listed? One of my test projects is in Dutch, which is not listed. Even if spellcheck is not possible because of a missing dictionary, I would think that some of the other checks (excluding jeebies) would be useful. Could an "Other non-English" option be added that would disable spellcheck unless a good words file is provided?

@asylumcs:

I think it's fine to run pptext if it's not a language on the list. Perhaps a user would not want to tick the "run spellcheck" box in some cases. And it's not really right to me to have a list anyway because there are perfectly good dictionaries loaded that do not have a checkbox. Wouldn't surprise me if Dutch were actually available.

[cpeel]

This is blocked on pptext itself working without a dictionary. After that issue is resolved we can revisit making the UI work without specifying a language / dictionary.