Various tweaks to the port matrix: open to full size; remove buttons and move their...

Various tweaks to the port matrix: open to full size; remove buttons and move their functionality into a context menu;
set maximum size hint to stop port matrix windows being resized beyond their useful size; remove visibility checkbuttons -
visibility now controlled through menu or by clicking on group names; port groups fold down to a small tab when hidden;
don't swap the orientation of the matrix once it has been opened.

git-svn-id: svn://localhost/ardour2/branches/3.0@5373 d708f5d6-7413-0410-9779-e7cbd77b26cf