gdb/tui: don't leak the known_window_types map

gdb/tui: don't leak the known_window_types map

This commit finishes the task that was started in the previous
commit.

Now that all Python TUI window factories are correctly deleted when
the Python interpreter is shut down, we no longer need to dynamically
allocate the known_window_types map in tui-layout.c

This commit changes known_window_types to a statically allocated data
structure, removes the dynamic allocation from
initialize_known_windows, and then replaces lots of '->' with '.'
throughout this file.

There should be no user visible changes after this commit.

Reviewed-By: Tom Tromey <tom@tromey.com>