form to upload a plugin jar in plugin tab

This is the initial commit for "enable plugin install/uninstall on the UI" feature. Work in Progress. Please feel free to give us feedback along the way. 
It's alright, to upload the same file name again, given that you use the DefaultPluginJarLocationMonitor. It takes care of it, by checking the modified timestamp (which can be wrong, but is usually ok). If it realizes that it is a changed plugin, it sends notifications to remove the older plugin from the registry and to put in the new one.

Since the plugin locations are on the filesystem, it's not possible to install two jars with the same name, twice. The second will overwrite the other.