Implement workspace stack

The idea is to import gnomonWorkspace into dtk. First we need a workspace factory and everything that comes with it.

We propose two ways to integrate workspaces into an app. The first one is to use a dtkWidgetsMenu that include a mechanism to choose the workspace (at the top level of the menu) (à la S2WD). The second one is to have at the bottom a global Worksapce bar (à la Gnomon) and each workspace has it's own dtkWidgetsMenu

• dtkWidgetsWorkspaceStackBar
• dtkWidgetsWorkspace abstraction
• dtkWidgetsWorkspaceFactory
• dtkWidgetsWorkspaceManager
• dtkWidgetsWorkspaceBar
• Example Main Window with a single dtkMenu that embed workspace chooser (See #6 (closed))
  • add dtkWidgetsMenuItemWorkspaceFacade
• Example Main Window with a single dtkWidgetsWorkspaceBar, each workspace has it's own dtkWidgetsMenu