[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.
I mean
```c
/// Iterates over all tabs in the tab list.
/// @param tp The identifier to use as the iterator.
```