Segment the shell into its own subparts

[IzKuipers]

The shell runtime is growing a lot with the upcoming fast window switcher, so it would be nice if each component of the shell (task bar, start menu, action center, calendar popup, workspace manager, fast window switcher) were its own "shell extension" that has its own class with its own component.

Each "extension" should set a "target" to indicate to ArcOS where in the shell the component belongs. For instance:

• Primary -- The top half of the shell that contains the start menu and action center, among other things
• Secondary -- The bottom stretch of the shell that houses the taskbar
• Global -- Neither primary nor secondary; just in the div.shell itself. This is stuff like the fast window switcher, current workspace indicator, etcetera.