Slideshow: Fix regression related to media animations

Misc effects tab page animations were lost.
Regression from
9695f38ca4ab26b40524f2eade98c45f0360131d.

Change-Id: I81f7321d7dd87f5b0f6b4acbecbd60241ab1e430
Slideshow: Remove unneded ExternalMediaShape

ExternalShapeBase is the base class of MediaShape and
AppletShape so it's nonsense that ExternalMediaShape
to be the base of ExternalShapeBase.
Actually this class does nothing, anyway.

Change-Id: I8a278c9273a1788f0859205b8f1d0905427179b6
Slideshow: remove unneded function duplicates

Change-Id: I682ae771f13e42dd1a4e1cf6a1ee062a5eca640f