DoubleTapped now can be triggered by touch
Fixed DoubleTapped triggering logic, previously it could be triggered only when you click twice, didn't work when you click 4 times,6, etc.
Fixed Tapped to be triggered only once when you double-click, now it triggers Tapped once and DoubleTapped once instead of triggering Tapped two times and DoubleTapped once.Matches UWP behaviour.
And no longer target net461 in unit tests as we've not had a test pass in net core and fail in netfx in years, and it bloats our CI time; EXCEPT Avalonia.Markup.Xaml.UnitTests where we're still testing against net4.7 as MSIL generation is runtime-sensitive.
Changed `MenuItem`'s `string InputGestureText` property to `KeyGesture InputGesture` and gave it a value converter in the default template, because the string representation of a key gesture depends on platform, and putting platform-specific behavior into `ToString` isn't a good idea.
To do this we needed to change the `KeyGesture.ToString()` method to produce input gesture text suitable for menus. Also demonstrated in `MainWindow` how to produce different gestures/headers depending on platform.
Added a new `IsEnabledCore` property to `InputElement` which is overridden in `Button` and `MenuItem` to override the `IsEffectivelyEnabled` state with the enabled state of the command.
Also add data validation of the `Command` property to `MenuItem` to make it behave the same as `Button` when `Command` is bound to a non-existent property.
Fixes#2501
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
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.
Tako
Steven Kirk
Giuseppe Lippolis
Dan Walmsley
José Pedro
Maksym Katsydan
Luis von der Eltz
Dariusz Komosinski
Nikita Tsukanov
Steven Kirk
sdoroff
Jeremy Koritzinsky