Add unit converter WPF sample app

[angularsen]

To illustrate how to enumerate quantities, units and to convert between them without hard coding any quantities.

The app lists all quantities (Length, Mass etc), and when you select one, it then lists all units for that quantity (if Length was chosen; meters, centimeters etc). Then it converts the numeric value from one selected unit to another.

This sample also highlights a limitation in the library that requires reflection in order to enumerate units for a selected quantity and to get the abbreviation for a unit, since the library does not provide a generic means to retrieve these without referencing types like Length and LengthUnit directly. There is definitely room for improvement in the library here, as previously discussed in #371 #354 #353 #308.

Fixes #274

[angularsen]

@eriove @ferittuncer @JKSnd for input. Also, this sample app pretty much sums up the current limitation of UnitsNet that currently requires reflection to do conversions without hard coded quantity types.

@YektaMirkan Since you asked about #354 Getting abbreviation list of a quantity
@dayewah Since you made a sample app and have been doing this dance already, this may be of interest to you.

[gojanpaolo]

Need null checking here?

public string FromHeader => $"Value [{SelectedFromUnit.Abbreviation}]";
public string ToHeader => $"Result [{SelectedToUnit.Abbreviation}]";

[0xferit]

I just started to examine this sample. I got an exception just after a clicking to a quantity.

NullPointerException at line 85 in MainWindowVm.cs.

[angularsen]

Thanks for testing it, I've added the null checks. I was not able to reproduce this behavior so likely a racing condition.

[0xferit]

Working flawlessly now.

I wonder why you didn't make result textbox editable like Google's converter.

[angularsen]

Ah, that was just me being lazy and trying to the the least amount of work :-)
I guess I just foresaw some issue with infinite cycles of one field change updating the other, which again updates the first one and so on. It's obviously solvable, I just avoided it the lazy bum that I am 😆

I'm merging in this since the bug is fixed. If anyone wants to improve this app, that is awesome too!