Until now, using a list as prefix/suffix didn't work as expected,
because the corresponding method for retrieving the value was called
multiple times, and each time, the next icon would be retrieved.
Changed the logic inside the theme to only update the indices every time
that an actual update was happening.
Add a way for themes to specify custom separators. Doing that, make
nicer interfaces for drawing "supplementary" components (separators)
for widgets and generalize the attribute retrieval within the theme.
