Backport Activation to RXUI 5.4

This is a back port to the 5.4 codebase of the new activation bits as described in #503. Not only super-useful for things like cancelling unnecessary HTTP requests, but also fixes some nasty memory leaks when used with WPF.

I appreciate that since the activation branch isn't complete this also isn't (specifically around inheritance), but I'll try to keep on top of tracking that branch and back porting. For now this is still useful as it resolves some memory issues in WPF.

## TODO:

- [x] Core activation stuff
- [x] XAML-specific activation bits
- [x] Android-specific activation bits
- [x] Cocoa-specific activation bits