SPARK-1888: Reformatting code for Plugin Manager
To fix the issue at hand, improving the readability of the code helps a lot. In this commit,
I've applied a code formatter (to just the Plugin Manager code) and made some basic changes
(added/improved log messages mostly).
SPARK-1888: Uninstall a plugin when the GUI instructs so.
This is a fix to what I assume to be a bug. The method, used by the UI part where plugins are managed,
is documented as 'remove and uninstall' a plugin. It only removed, but did not uninstall. I've fixed that
with this commit.
