Media classes should be compiled with -target 1.8

This will probably be taken care of as a natural side effect of switching to gradle for compiling the media classes, but worth tracking anyway.

While discussing https://jbs.oracle.com/bugs/browse/JDK-8023279 it was pointed out by David Holmes that there are several classes with version 49 (Java 1.5 class format) and that they will load more slowly. All FX classes should be version 52 (JDK 8), except for the builder classes which are version 51 (JDK 7) as required by the fix for RT-30518.