Several small changes

[cpiber]

Each of these is pretty small, so I hope it's alright to put them in one PR.

• Add [Parameter] to OnErrorReceived
• Change async void to async Task and await everything
async void is evil
• Catch exceptions in all internal operations
  These are not catchable by the library user, we should be careful
• Make useful functions/members protected
  This allows easier subclassing, e.g. someone might want to change the rendering of the reader completely
• Bind SelectedVideoInputId and provide change events
• Hook up disposing events and remove listeners on dispose
• React to DecodedPictureCapture parameter change

Note: I noticed that JS-.NET interaction is a bit awkward, might want to look into instance calls and interop integration.

Closes #54, #53,

[sabitertan]

Can you please update Readme according to your changes? TY!

[cpiber]

Btw, unrelated to my PR, but I think the section "ImageCapture Support" is a bit misleading. It should be mentioned, that this is only needed for the capture function, not for the library as a whole. I did not even notice any problems when I used the library despite using Firefox, since I did not need the feature.