Droid: Define non-async methods in terms of async methods, not vice versa. This is because a TaskCompletionSource can keep track whether it has been completed, whereas with normal callbacks we'd have to implement a check by ourselves. This is important for the dismiss-after-time feature I'm about to implement.
Droid and portable: Set default values of all Action-methods to null.
iOS: Set default values of all Action-parameters to null.
Added TimeSpan-parameters that dismiss a dialog after some time. Defaults to null. Implemented in Droid, ignored in iOS so far.
Divide Input-methods into InputText and InputNumeric. Implemented on Android. NotImplementedException on iOS since I have absolutely no insight into iOS development.
Fix: Do not try to parse decimal if the string to parse is null or empty.