Header cleanup: try to include as little as possible; this will probably speed up compilation a bit.

Original commit message from CVS:
Header cleanup: try to include as little as possible; this will probably
speed up compilation a bit.
changed the .c files to use #include "..."
Fix for the 'plugins are loaded twice' bug.
Fix 22186: GstObject flags are now used everywhere. Added *_FLAG_LAST so
elements do not use the same flags. Added some padding in the flag enum
for future expansion.