Creates a new DesktopAppInfo based on a desktop file id.
A desktop file id is the basename of the desktop file, including the .desktop extension. GIO is looking for a desktop file with this name in the `applications` subdirectories of the XDG data directories (i.e. the directories specified in the `XDG_DATA_HOME` and `XDG_DATA_DIRS` environment variables). GIO also supports the prefix-to-subdirectory mapping that is described in the Menu Spec (i.e. a desktop id of kde-foo.desktop will match `/usr/share/applications/kde/foo.desktop`).
the desktop file id
a new DesktopAppInfo, or null if no desktop file with that id exists.