Blender Git Loki

Cleanup: Main id looping: add FOREACH_MAIN_LISTBASE macro.

We don't want to use flow control like `break` statement into the basic
`FOREACH_MAIN_ID` macro, as this is a nested loop.

When refined behavior is needed (like breaking whole iteration, or just
skipping to next ID type), FOREACH_MAIN_LISTBASE and
FOREACH_MAIN_LISTBASE_ID macros should be used instead.

Based on D4382 by @campbellbarton
(Other potential solution, using flow control macros: D4384).