Fix case sensitivity for the spell checker.

The new behavior is as follows:
- If the word in the dictionary is not fully lower case, then the
exact case is required to match.
- If the word in the dictionary is fully lower case, then any of
the following patterns match:
  - fully lower case
  - only the first char capitalized
  - all caps
Any other capitalization is rejected.

This is probably what people want. If you type a name in all lower
case, it should be marked as a typo, but if you type a word with a
capital for emphasis or just because it's the start of the sentence,
it should match a lower case word in the dictionary. If you have
a spurious capital letter in the middle of a word because of a typo,
it should be marked as such.

Accents are not affected, and should not be. An accented letter
is a different letter and a missing accent should be reported.
We should maybe consider again for some common transpositions
like the "ue" digraph for German, which is now considered a typo,
but will suggest the correct diacritics as the first suggestion.

Bug: 5145751
Change-Id: I651e24f13c90fb94700a1674ad380e95336e7dca