15 no ability to use a custom icon

[kcl93]

Hi @evanwon I took some time to make a much larger pull request this time. In general I tried to have the project compliant with the MVVM style. In general I have created viewmodel and model classes and linked them together. Users that might want to have more flexibility, can use the MessageBoxModel class directly. This one allows users to configure the messagebox much more freely. Also it allows users to use the known Show() and ShowDialog() methods. If Show() is being used it is even possible to update all texts in the MessageBox while it is still open. This way you could for example add a countdown to the message box.

[kcl93]

This PR should address the issues 6, 12, 13, 15 and 17 (maybe partly 5 too since you now can increase the maximum button width)

[kcl93]

The next two topics that I would like to tackle (separately) are the localization issue and I would also like to update the project files in order to have nuget packages being autogenerated during build (and maybe more supported frameworks).