Refactor all public static DockNotebook functions into a static class called DockNotebookManager, and add some convenience methods for work we had in MainWindow.
Refactor all public static DockNotebook functions into a static class called DockNotebookManager, and add some convenience methods for work we had in MainWindow.

Also update behavior:
- New tabs will be added to the active container rather than the first container.
- New tabs will be added to the right of the active tab rather than the end.

Mailaender/Pinta,Mailaender/Pinta,PintaProject/Pinta,jakeclawson/Pinta,jakeclawson/Pinta,Fenex/Pinta,PintaProject/Pinta,Fenex/Pinta,PintaProject/Pinta