app: add an "interactive" boolean inside gimp_export_image()

instead of using "format_name", which is confusing, and file plug-ins
will soon pass their format name again.

(cherry picked from commit 206df0026654c7d2a1eff317a02e86e317e6ae80)
Bug 705457 - Export doesn't handle a single top-level layer group

Add special export code for single top-level layers groups, other
cases should be mostly (not perfectly) handled by the export code
for layered images. Also, show the legacy export dialog when the
envorinment variable GIMP_INTERACTIVE_EXPORT is set, so export
changes can actually be tested and verified again.

(cherry picked from commit 8c79a070bb28416752d76c4f8505119871819bfb)