Pull Commands Into Own Assembly

[benrick]

This is not a complete transition to having commands be a plug-in, but is intended as a stepping stone to that goal.

Notes:

• Changes MainMenuItems to have a state collection to load/unload plug-ins into/from.
• Adding validation rules to the CustomCommandValidator.
• CustomCommands get validated by the CustomCommandValidator (not using DI)
• Connects validation to the commands datagrid.
• Only saving commands that pass validation.
• Icon type simplified to avoid dependency on the icon packs from MahApps.

Issues:

• The custom validation for the commands is not injected.
• Wire up of navigation through click event.
• Had to put the *Views as Content of the menu items.

[SimonGeering]

Code clean-up bits and bobs :

• Need to sort out a standard naming convention for DependencyInjectionExtensions.cs vs DependencyInjection.cs

New issue needed for things the PR has brought up that we need to look at later:

• Define a boundary when loading plugins into the system that validates plugin state on the assumption the plugin author messed up somehow.
• Establish a naming convention for plugins/plugin assemblies.
• Architectural split between in-box plugins and plugin sdk bits.
• Decide if we should kill off the Core namespace.
• Review file names and namespace consistency more generally
• Error handling infrastructure
• Logging infrastructure

[SimonGeering]

Looks good to go, we are on the way :-)