TextDialog: Improve the average and worst-case performance of word wrapping.

This affects manuals, About GMenu2X, and the Log Viewer.

Instead of trying to compute the width of the entire string, then backing
off one word at a time, TextDialog::preProcess now performs a binary search
on Font::getTextWidth(string) and backs off to the last fitting space, if
there is one, at the last moment.

In Japanese and Chinese text, words are not usually separated by spaces.
Text in these languages is now wrapped when it would reach the edge of the
screen.