Now that the Docks API is shipped, the Workspace's active item can be either a pane item in the Workspace center or an item in one of the three Docks. This includes items which were previously Panels, like the tree view or our own Git- and GithubTabControllers. As none of these implement the "active context" pane API, moving focus to any of these causes the context resolution algorithm to fall back on the added projects, which doesn't always work as expected if there are zero or many projects in the workspace.

This will fixes #825 and issues like it in several layers by:

• Inferring the active context from an active item in the Workspace center if the active pane item doesn't implement the context API
• Implementing the context API for the Git and GithubTabControllers so that clicking on one of our own dock items never causes an active context switch