new ArdourButton class, to start to provide more control over how our buttons work...

new ArdourButton class, to start to provide more control over how our buttons work and appear; use ArdourButtons for monitor, solo isolate and solo safe buttons, and in the processor box; don't save UI config file (canvas colors) to the user's home dir unless one or more parameters were modified by the user

git-svn-id: svn://localhost/ardour2/branches/3.0@10311 d708f5d6-7413-0410-9779-e7cbd77b26cf