Implement word wrapping as a method of Font.

This implementation is based on the implementation in TextDialog::preProcess,
with one major difference: it works on the entire input string, copies it much
less as part of its function, and tries to quickly establish a small search
space for the length of the beginning split of each line.

With most standard fonts and sizes, this means up to 9 computations of metrics
per output line.
Use Font::wordWrap in TextDialog and TextManualDialog.