Activate ellipsization for the style combo in the export dialog.

This ensure that the dialog will not goes out of the main window.
Add ellipsization code into gtk.c for sharing purpose.

Code clean-up.
Use dt_ellipsize_combo() where needed.

And remove hard coded widget width.
Merge branch 'fix-export-style-width' of https://github.com/TurboGit/darktable