MADS: Beginnings of work on Rex Nebular main menu
MADS: Fixes for compilation warnings
MADS: Remove redundant saved surface from FullScreenDialog base class
BUILD: Fix AmigaOS4 build variable naming to be clearer.
BUILD: Change AmigaOS4 packaging target to "amigaos4dist".

This is a clearer naming, but needs update to buildbot to support.
AmigaOS4 Porter (raziel_) is already aware of this.
BUILD: Add port specific makefile for AmigaOS4.

This moves the AmigaOS4 specific packaging rules from the global
ports.mk to a port specific makefile in the AmigaOS subdirectory of
the SDL backend (used for AmigaOS).

Previously, port specific makefiles were only used for cross compiling
builds and thus had a single entry in the cross compiling section of
the configure. Since AmigaOS4 is a desktop system supporting native
build, this required a second entry for when the host system is detected
as ppc-amigaos to support native builds.

However, currently this does break packaging of cross compiled builds
for Win32, OSX and Unix when done on AmigaOS4... but this is not likely
and has limited impact. To fix this, default _port_mk lines would need
to be added to the sections of the cross compiling switch for mingw32
etc. to override the AmigaOS4 OS setting of _port_mk.
MADS: Initialise _lineIndex

Without this, Rex Nebular crashes on startup for me.
SAGA: Move and rename some defines which are only used in the ITE intro
SAGA: Fix spacing
SAGA: Get rid of the MUSIC_DEFAULT flag
SAGA: Add wrappers for queuing music events
SAGA: Move the ITE intro text together with the other static ITE data
SAGA: Move ITE credits together with the other ITE static data
SAGA: Merge all of the ITE intro cave scene handlers
SAGA: Remove the now unneeded individual cave scene handlers
SAGA: Some renaming
SAGA: Add debug commands for playing music, sounds and voices
SAGA: Cleanup
SAGA: Add support for MIDI music in SAGA2 games
MADS: Fix palette method signature