Add a theme-specific predefined list of css classes

This pull request adds support for picking css classes from a
theme-specific list. This list is defined in a text file under the
theme directory (called block-styles.txt). The classes listed in this
text file (each class is separated by new line or tab or spaces) may be
chosen from the "Set Block Styles" dialog.

I made this function because one of our themes defines css classes that
users have to choose from, but they must write it down by hand because
they always forget them. This improvement helps saving paper ;)