Add a preferences page for user settings

[ymeine]

Externalize some data as user preferences.

For now everything in the plugin is statically configured (it is often present as final static variables in the source code). There's no command line input, neither any possibility of configuration file input.

There should be a preferences page, a standard one like those we see in other Eclipse plugin (integrated to the preferences management of the whole application).

There are several kinds of things that can be configured:

• things related to the communication (and possibly integration) of the backend
• things related to desired behavior of the backend - configuration of the services
• things related to the plugin itself solely

This list is not exhaustive, neither is the following of suggested configurations: d:

• External backend re-use
  • enable external backend re-use or not
  • port on which the external backend is running
  • GUID identification pair: in case this changes on Backend side, no need to re-deploy a new version of the plugin with new static values, just configure it
• Backend program management (that is the plugin launches and manages an instance of the backend)
  • use embedded backend or external installation: even if the plugin can launch a backend, it can be desired to use an external, possibly shared, instance of the backend
  • path of the external installation: in case we don't want to use the backend source embedded in the plugin
  • use system Node.js installation (the one in the PATH) or not (migth require a specific version)
  • custom Node.js path in case we want to use one
  • port to use when launching a backend
  • enable to find and use first available port (some additional data might be required for that, like a range, ...)
  • launch timeout before fallback

Please complete this list!