Issue #70.

Plugins with userlist entries and unsaved edits have their names bolded
in the plugins list.
Also fixed a bug with dirty info not being cleared when changing plugin.
Fixed bug where file display fields would be generated needlessly.
Issue #70 work.

* Fixed range limits on the priority spinner.
* Implemented right-click menu for plugin list, moved "Copy Metadata As
Text" into it and added "Copy Name" and "Remove All User-Added Metadata"
entries.
Updated readme for UI changes.