Multitargeting NetCore 3 and Net 4/Net 4.5.2

v1.5

Consolidated namespace in App.xaml.cs and App.xaml

Theming Integration for MLib

v1.4.1

Merge branch 'master' of https://github.com/Dirkster99/MsgBox

Released Version 1.4.0 from Codeplex

This is the same version as released from Codeplex at: https://msgbox.codeplex.com/
except that we are now managing sub-system components (UserNotifications) via nuget.