[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.
Even macros can use doxygen-style comments. In fact, such documentation, and with a remark about the `tp == curtab` check might be helpful for `FOR_ALL_WINDOWS_IN_TAB`.