Opening Phoebus Help in the default web browser instead of in the JavaFX WebView

[abrahamwolk]

Currently, when clicking on the menu item 'Help' -> 'Help' in the menu bar of Phoebus, the Help is opened in an instance of the JavaFX WebView inside Phoebus. We would like to change this behavior, so that the help files are instead opened in the default browser of the operating system.

This can be accomplished by changing the action on the instance of the MenuItem of 'Help' to launch the default browser instead of creating an instance of HelpApplication.

Concretely, the proposal is:

1. To change the action on the instance of the MenuItem of 'Help' to launch the default browser instead of creating an instance of HelpApplication.
2. To remove the HelpApplication from Phoebus.

[kasemir]

Makes sense to launch a plain browser instead of a web view.
But I would keep the logic in https://github.com/ControlSystemStudio/phoebus/blob/master/core/ui/src/main/java/org/phoebus/ui/help/HelpBrowser.java to determine what to show: If we have help bundled as "doc/", open that, otherwise fall back to the readthedocs copy on the web.