ActivityLauncher requires ActivityID in creator

ActivityControl can return more than one ActivityLauncher
in the  ActivityControl.Launchers property

ActivityControl.Launcher property removed.
Added Sales Manager plug-in project on a new a new branch.
Added string.IsNullOrEmpty(str) extension method.
Database implementation via XpressData.

 - Plug-in gets database path from settings.
 - Prompts for database as fallback method.
Python-style string.Join() extension method
Partial implementation of client list

 - Load & display clients
 - Delete clients
Updated to reflect changes to ActivityControl.Launchers property
Fixup!

Updated to reflect changes to ActivityControl.Launchers property
Merge branch 'SalesManager' of https://github.com/franzalex/ExtensibleFramework into SalesManager