Don't fix size of preset combo any more. Comment what

Don't fix size of preset combo any more.  Comment what
widgets the PlugUIBase makes available for subclasses.
Give up on trying to report when a VST plugin's settings
have been modified from a preset, since I think it is
impossible to do cleanly (you set the preset, then at some
point after that the plugin tells you that values have
changed using the same mechanism that it uses to report
normal user-initiated changes).

git-svn-id: svn://localhost/ardour2/branches/3.0@10798 d708f5d6-7413-0410-9779-e7cbd77b26cf