[wayland] Keep application startup flow inside main_wayland

Summary:
In a recent patch the newly added xwayland class ended up being
responsible for continuing the startup, calling back into the main app
to spawn the workspace.

It moves the flow of startup about so it's not very readable or
following class structure.

This patch moves the code back into main_wayland and removes the
duplication between xwayland and non-xwayland modes.

There was also a misnaming of methods.

Previously:
continueStartupWithScreens was called after platform screens are created
continueStartupWithScene was called after the scene was created
continueStartupWithXwayland was called before xwayland is created

This was confusing, so the names have been shuffled around to follow a
consistent pattern of what has been done so far.

Test Plan:
Started kwin_wayland in normal and xwayland mode
Ran unit tests (though some failed due to a local unrelated and as yet unindentified bug)

Reviewers: #kwin, romangg

Reviewed By: #kwin, romangg

Subscribers: romangg, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D19232