Use desktop files for default aplications

When changing the default application in one of the primary categories (web browser, file manager, etc.) to a program not shipped by default (in the helpers directory), xfce-mime-settings only presents a single text entry to enter the command-line. This is different than its behavior for ordinary mime types, where it first presents a list of all applications (from desktop files), and presents the command-line as an advanced option. The latter is preferable as it is much simpler for the typical use case of selecting an app already installed on the system.