gdb/tui: Place window titles in the center of the border

gdb/tui: Place window titles in the center of the border

In tui-wingeneral.c:box_win () a comment suggest we should display
titles like this:

  +-WINDOW TITLE GOES HERE-+

However, we actually display them like this:

  +--WINDOW TITLE GOES HERE+

The former seems nicer to me, so that's what this commit does.  Short
titles will appear as:

  +-SHORT TITLE------------+

We previously didn't test the horizontal windows borders in the test
suite, however, I've updated things so that we do now check for the
'+-' and '-+' on the upper border, this will give us some protection.

gdb/ChangeLog:

	* tui/tui-wingeneral.c (box_win): Position the title in the center
	of the border.

gdb/testsuite/ChangeLog:

	* lib/tuiterm.exp (Term::_check_box): Check some parts of the top
	border.

Change-Id: Iead6910e3b4e68bdf6871f861f23d2efd699faf0