[FEATURE] Add getters for different media elements

There are 3 new getters to get the specific items directly. This is
especially useful to use e.g. <f:count> on those items.

=> newsItem.mediaTypeImage
=> newsItem.mediaTypeMultimedia
=> newsItem.mediaTypeDam

Change-Id: I2b461de35986eeb520079f197ecb2f7d483bbb95
Reviewed-on: https://review.typo3.org/24141
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer