Move all GIMP stock icons into dedicated folder under original filename

Stock icons copied from GIMP are scattered across random folders with random filenames. This makes maintenance extremely cumbersome, e.g. when GIMP's default icon theme changes. One outcome of this situation is having some GIMP stock icons copied up to four times into the gimp-help repository, under different filenames.

Thus move all stock icon image files into a dedicated folder and rename the files to be in sync with the filenames in GIMP itself in order to make maintenance more future-proof (and to also allow translators to realize via the path exposed in the .po file that no localization of these image files is needed).

In a future step, actually replace/update the current stock icon files in gimp-help with the icons actually used in GIMP (as many of the image files moved and renamed in this patch still depict the deprecated legacy icon theme of GIMP).

See #435