Add TrailingIcon property

## 🚀 Feature Requests

We should have the option to create a custom trailing icon.

### Contextualize the feature
It would be useful to add a custom trailing icon for something like a password field. There could be a padlock icon at the end of the field which reveals the password when it is tapped.

### Describe the feature
For this feature to work there are a few things required:

1. TrailingIconProperty
2. TrailingIconCommandProperty
3. TrailingIconCommandParameterProperty

All of this should be bindable to an ICommand too.

### Platforms affected (mark all that apply)
- [x] :iphone: iOS
- [x] :robot: Android
- [x] :checkered_flag: WPF
- [x] :earth_americas: UWP
- [x] :apple: MacOS
- [x] :tv: tvOS
- [x] :monkey: Xamarin.Forms