Convert NetDaemonApp to abstract class

[helto4real]

Breaking change

If you are using the NetDaemonApp class directly you now have to make a derived class to use it.

Proposed change

The NetDaemonApp should be an abstract class. The design intent is not to use it directly. Each app in NetDaemon should inherit from it.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes Make NetDaemonApp.cs an abstract class #89

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the [development checklist][dev-checklist]
• The code compiles without warnings (code quality chek)
• [] Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io