gst/gstpad.c: check that caps are fixed

gst/gstpad.c: check that caps are fixed

Original commit message from CVS:
* gst/gstpad.c: (gst_pad_set_explicit_caps):
check that caps are fixed
* gst/gstpad.c: (gst_pad_template_new):
don't try to simplify caps, costs too much time on gst_init
* gst/gstplugin.c: (gst_plugin_add_feature):
G_ERROR if features are added twice
* gst/gsttypefind.c: (gst_type_find_register):
* gst/gstelementfactory.c: (gst_element_register):
don't add features twice
* docs/random/ds/0.9-suggested-changes:
add note about possible gst_init optimization