[RFC] FOR_ALL_TAB_WINDOWS cleanup. Add FOR_ALL_TABS and FOR_ALL_WINDOWS_IN_TAB.

Adds `FOR_ALL_TABS` and `FOR_ALL_WINDOWS_IN_TAB`. These combine together to make the `FOR_ALL_TAB_WINDOWS` helper that already existed.
What do you mean by doxygen-style? I'm guessing you mean to write a multiline `/* ... */` comment with the variables documented and such. Just want to make sure I understand correctly.