Add comments describing tui_ui_out and its fields, cleanup a bit

Add comments describing tui_ui_out and its fields, cleanup a bit

This commit add comments describing tui_ui_out and its fields, and
cleans up the code a little bit.

Also switch to using in-class initialization so that the initial
values can be seen alongside the comments.

I see no reason for initializing m_line as -1 instead of 0, since all
the checks in the .c file are of the form "> 0".  AFAICS there's no
practical difference between -1 and 0.  So it seems simpler to
initialize it as 0.

There's a bit of redundancy in tui_ui_out::do_field_string, which is
fixed by this commit.

gdb/ChangeLog:
2019-03-19  Pedro Alves  <palves@redhat.com>

	* tui/tui-out.c (tui_ui_out::do_field_string): Simplify.
	(tui_ui_out::do_text): Add comments.  Reset M_LINE to 0 instead of
	to -1.  Fix TABs vs spaces.
	(tui_ui_out::tui_ui_out): Don't initialize fields here.
	* tui/tui-out.h (tui_ui_out) Add intro comments.
	<m_line, m_start_of_line>: In-class initialize, and add describing
	comment.