Tutorial 4 - no webpage inspector

## Expected Behavior
I've been told that it should be possible to run the webpage inspector from the right-click menu.

## Current Behavior
In the right-click menu there are no commands that would run page inspector.

## Your Environment
I am running Toga on Ubuntu 16.04 with Anaconda Python. Virtual environment is made with ```conda``` instead of ```virtualenv```.

* Python Version - ```3.6.8``` from ```--conda-forge``` channel

* Operating System and Version (select from the following and list the specific version number; if your OS is not listed, list that as well)
- [ ] Linux - distro: - version: Ubuntu 16.04

* Toga Target (the type of app you are trying to generate)
- [ ] gtk

No source file or test is named. Start by reproducing the missing right-click commands for the GTK target on the reported environment, then trace where Toga builds that menu. Done means the webpage inspector commands appear in the right-click menu and launch the inspector successfully.

Written by the indexing model from the issue text.