Port Activation core stuff from RX6 to 5.4
Port XAML-specific activation from RXUI6 to 5.4
Port Android-specific activation from RXUI6 to 5.4
Port Cocoa-specific activation from RXUI6 to 5.4
Backport inheritance handling to RXUI 5.4
Merge remote-tracking branch 'jen20/activation-5.4'
Fix double activation in WPF

This is the same fix applied to RXUI6 as in #526, backported to RXUI 5
with activation
Allow templates for TCC not to have previous site

- Remove the null check on "PART_PreviousContentPresentationSite" when
  applying the template
- When enqueuing a transition test whether a previous presentation site
  exists in the template, if not set the content directly without any
  transition.
Merge remote-tracking branch 'origin/activation-5.4'