Support clearing bulletins from processors via right-click context menu

Since most bulletins of error or warning type are lengthy, it takes only 2-3 bulletins to cause display to extend below the bottom of the screen.

It's common to iteratively edit processor configuration and test for desired effect. When the desired setup is finished, processors will continue displaying bulletins for 5 minutes, leading users to be unsure whether new errors/warnings are coming in, or whether they're old and can be ignored.

NiFi's UI should support "clearing" bulletins from processors via a right click menu.