Bug 1545342 - part 1: Create mozilla/PresShellForwards.h and move global enums/constants in nsIPresShell.h and mozilla/PresShell.h into the new one r=smaug

This patch creates new header, `mozilla/PresShellForwards.h`.  It should have
all forward declarations of global class/struct in `nsIPresShell.h` and
`mozilla/PresShell.h`.

Additionally, this moves all `enum`s and `constant`s in them into the new file
with changing them to `enum class`es.

This will make other headers which require only specific types in the header
files not include them.

Differential Revision: https://phabricator.services.mozilla.com/D28605

--HG--
extra : moz-landing-system : lando