Issue #9410: Show GIMP icons rather than GTK defaults

This patch attempts to resolve issue #9410 (closed). Due to the file naming conventions, GTK was using default icons for "document-new", "folder-new", "edit-delete", "go-up", and "go-down" rather than the custom icons created for GIMP's Symbolic and Color icon themes.

To fix this, "gimp-" prefixes were added to the icon files and updated in libgimpwidgets. A few places that hardcoded references to "edit-delete" were replaced with GIMP_ICON_EDIT_DELETE to make any future changes to the filename easier.