switch visibility of scrollers from private to protected to allow subclassing in Navigator plugin to deal with negative slice indices.

Due to this and other hacky shortcuts to get the navigator display working, an extra copy of all image display classes will likely need to be copied into the plugin once a stable version is ready.

git-svn-id: https://valelab.ucsf.edu/svn/micromanager2/trunk@14076 d0ab736e-dc22-4aeb-8dc9-08def0aa14fd
Allow zooming to 33% and similar not-easily-expressed-as-decimal levels.

git-svn-id: https://valelab.ucsf.edu/svn/micromanager2/trunk@14077 d0ab736e-dc22-4aeb-8dc9-08def0aa14fd
Make plugin loading slightly more resilient to partial failure.

If a single directory fails to load, don't give up on loading plugins
altogether; continue with loading the others (and log an error of course).

git-svn-id: https://valelab.ucsf.edu/svn/micromanager2/trunk@14078 d0ab736e-dc22-4aeb-8dc9-08def0aa14fd