Remove ResourceSelectorHost and clean up/refactor related bits (#3050)

* Sever dependency on legacy resource selector host

Audio resource selectors (browse dialogs) no longer need to be
registered with the legacy IResourceSelectorHost system.  Set up a new
EBus specifically to handle browse button presses and directly invokes
the dialog.

Signed-off-by: amzn-phist <52085794+amzn-phist@users.noreply.github.com>

* Hook up legacy audio control selector to new EBus

Remaining use of legacy audio selectors (trackview) need to be able to
bypass ResourceSelectorHost now.

Signed-off-by: amzn-phist <52085794+amzn-phist@users.noreply.github.com>

* Removes ResourceSelectorHost and legacy selectors

This removes various Variable types that were tied to resource
selectors, such as GeomCache, Model, Animation, File.  Removes the
ResourceSelectorHost completely.  The two things that still appeared to
have selectors in TrackView are Audio Controls and Texture.  Fixed the
audio control selector to work via EBus and the Texture selector didn't
seem to work at all, but left it in as it was.

Signed-off-by: amzn-phist <52085794+amzn-phist@users.noreply.github.com>

* Make the default audio selector return old value

Signed-off-by: amzn-phist <52085794+amzn-phist@users.noreply.github.com>

* Fix some signed/unsigned comparison warnings

Signed-off-by: amzn-phist <52085794+amzn-phist@users.noreply.github.com>

* Remove deleted function from Editor Mock

Signed-off-by: amzn-phist <52085794+amzn-phist@users.noreply.github.com>

* Change audio selector api to use string_view

Per feedback.

Signed-off-by: amzn-phist <52085794+amzn-phist@users.noreply.github.com>

Code/Editor/Util/Variable.h

@@ -142,15 +142,10 @@ struct IVariable
         DT_PERCENT,     //!< Percent data type, (Same as simple but value is from 0-1 and UI will be from 0-100).
         DT_COLOR,
         DT_ANGLE,
-        DT_FILE,
         DT_TEXTURE,
-        DT_ANIMATION,
-        DT_OBJECT,
         DT_SHADER,
         DT_LOCAL_STRING,
         DT_EQUIP,
-        DT_REVERBPRESET,
-        DT_DEPRECATED0, // formerly DT_MATERIAL
         DT_MATERIALLOOKUP,
         DT_EXTARRAY,    // Extendable Array
         DT_SEQUENCE,    // Movie Sequence (DEPRECATED, use DT_SEQUENCE_ID, instead.)
@@ -160,7 +155,6 @@ struct IVariable
         DT_SEQUENCE_ID, // Movie Sequence
         DT_LIGHT_ANIMATION, // Light Animation Node in the global Light Animation Set
         DT_PARTICLE_EFFECT,
-        DT_GEOM_CACHE, // Geometry cache
         DT_DEPRECATED, // formerly DT_FLARE
         DT_AUDIO_TRIGGER,
         DT_AUDIO_SWITCH,