Issue #788: also install all python plug-ins in their own directory.

It seems I forgot to also install python plug-ins in a subdirectory
(because they are deactivated on master for the time being). I even
found a year-old patch from myself rotting in the tracker, and which was
doing exactly this.
I redid it, improved to fit changes.

plug-ins/pygimp/plug-ins/Makefile.am

@@ -3,28 +3,31 @@
 pluginexecdir = $(gimpplugindir)/plug-ins
 
 scripts = \
-	colorxhtml.py			\
-	file-openraster.py		\
-	foggify.py			\
-	gradients-save-as-css.py	\
-	histogram-export.py     	\
-	palette-offset.py		\
-	palette-sort.py			\
-	palette-to-gradient.py		\
-	py-slice.py			\
-	python-eval.py
+	colorxhtml/colorxhtml.py			\
+	file-openraster/file-openraster.py		\
+	foggify/foggify.py				\
+	gradients-save-as-css/gradients-save-as-css.py	\
+	histogram-export/histogram-export.py     	\
+	palette-offset/palette-offset.py		\
+	palette-sort/palette-sort.py			\
+	palette-to-gradient/palette-to-gradient.py	\
+	py-slice/py-slice.py				\
+	python-eval/python-eval.py
 
 test_scripts = \
-	benchmark-foreground-extract.py	\
-	clothify.py			\
-	shadow_bevel.py			\
-	sphere.py			\
-	whirlpinch.py
+	benchmark-foreground-extract/benchmark-foreground-extract.py	\
+	clothify/clothify.py						\
+	shadow_bevel/shadow_bevel.py					\
+	sphere/sphere.py						\
+	whirlpinch/whirlpinch.py
 
-pluginexec_SCRIPTS = $(scripts)
+%.py:
+	mkdir -p $(@D) && cp -f $(srcdir)/$(@F) $@
+
+nobase_pluginexec_SCRIPTS = $(scripts)
 
 if GIMP_UNSTABLE
-pluginexec_SCRIPTS += $(test_scripts)
+nobase_pluginexec_SCRIPTS += $(test_scripts)
 endif
 
 # python-console has a data file.