gdb/tui: change some macros to functions

gdb/tui: change some macros to functions

Change the `TUI_*` macros to access known windows to functions.  Define
them in their respective files, because trying to define them in
tui-data.h would end up causing include cycles.

This makes static analysis (detection of unused include files in this
case) more accurate, and I think in general we should avoid hiding
code behind macros if not necessary.

Change-Id: I1e38cee843984c48ab34030b19dac0d726f851af
Approved-By: Tom Tromey <tom@tromey.com>