IBaseEntity and IBaseRepository added. Unstable.

Significant step towards making the controls and commands generic. A few TODO's added. All entities now implement the IBaseEntity interface, corresponding repositories - fixed.
TODO: All other repositories should implement the IBaseRepository interface.
TODO: Commands (new, edit, delete) should be made generic.
TODO: EntityControllers should be written for main entities - abstraction layer over repositories and forms.
TODO: DataBrowserControl when finished will be able to display and edit all types of entities.