Measure: editing environment variables

[MatthieuDartiailh]

As we allow environment variable to be anything that can be pickled (this need to be very explicitly said in the docs), we need to provide matching widget to edit their values. As this is something likely to grow with people imagination we need an extensible way of providing editors. We of course also need a widget present in the measure workspace to edit the values, and we need to be able to save/load environments. The following notes described the necessary work:

• Providing editors:
  • editors should be provided to an extension point on the measure plugin.
  • all editors should inherit from a common base class (inheriting from Container as we will always need to display at least the id and the value)
  • editor should be contributed through a dedicated object in charge of instantiating the widget and with two declarative method used to format the value before saving and restoring the value from config. By default those methods should simply use repr and literal_eval
• Implementing the edition widget:
  • the edition will be a DockItem anchored by default in top tabbar
  • this DockItem should contain a TreeWidget displaying the EnvNode and EnvVars, next to it we should display all the EnvVars and nodes below the currently selected one. If an EnvVar is selected we should display the node entries. Editors should be cached just like task views.
• Adding menu to the workspace:
  • to add entries to the menu some new contribution should be added to the workspace manifest