This change adds the option for bigram prediction under the option
for bigram suggestion. The "prediction" option depends on the
"suggestion" option.
This change also reorders a tad bit the settings screen. Namely, it
sticks both bigram options under a "bigram" category, and groups the
options that did not have a group under a "other options", or misc,
category.
Finally this change also renames the internal name of the "text
corrections" option category to "correction_category" instead of
"prediction_category" which was misleading.

Change-Id: I2d8787c8a391fd8202ed3a686c613494b7260a1e