Returns true if some text is selected; places the bounds of the selection in start and
end (if the selection has length 0, then start and end are filled in with the same value).
start and end will be in ascending order. If start and end are NULL, then they are
not filled in, but the return value still indicates whether text is selected.
| this |
a TextBuffer a TextBuffer |
| start |
iterator to initialize with selection start |
| end |
iterator to initialize with selection end |
|
whether the selection has nonzero length |