Previously it wasn't clear which constructor on `InstancedBinding` must be called for a particular binding mode. Refactored the constructors:
- We now have a single public ctor which takes an `ISubject`, as a subject can be used for all binding modes.
- Instanced bindings with other sources such as single values and `IObservables` are now constructed via static methods which only allow the correct sources for that binding mode
When a captured control is removed from the visual tree, clear the mouse capture.
Note that our capture logic needs to be improved - it shouldn't be possible to capture a non-visible or non-enabled control. This doesn't address that but it does fix the crash in #1203.
Fixes#1203
Update Rx.Net dependency to match reactiveui's dependency on Rx.
Removed ReactiveUI dependeny in BindingTests.
Updated BindingTest sample to use the new reactiveui APIs.
Update samples to use current ReactiveUI APIs.
Fixed bug I introduced into tests.
Remove RxUI submodule info from .gitmodules
Previously this project was spewing loads of
```
Severity Code Description Project File Line Suppression State
Warning CS1701 Assuming assembly reference 'System.Runtime, Version=4.0.20.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' used by 'Avalonia.Controls' matches identity 'System.Runtime, Version=4.1.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' of 'System.Runtime', you may need to supply runtime policy Avalonia.Input.UnitTests(net461) D:\projects\Avalonia\tests\Avalonia.Input.UnitTests\CSC 1 Active`
```
Warnings. Not sure why this was, but updating the `csproj` to be in the same format as the other unit tests stops these warnings and reduces the warnings for a solution build from 5755(!) to 238.
Steven Kirk
Jeremy Koritzinsky
Nikita Tsukanov
José Pedro
Dan Walmsley
Jurjen Biewenga
Steven Kirk
Eli Arbel