native dark look

Dark theme styling from other plugins such as JDT, PDE, EGit are not
applied to e4_dark_mac1013 theme.
The theme id of the e4 dark theme should be
org.eclipse.e4.ui.css.theme.e4_dark only. Other components depend on
this theme id to provide the styling.

Added a check in Theme Engine to register only one css file for the e4
dark theme based on the os_version.

Change-Id: I37438012300e0ad54cdab7709caf4bfd2199def5
Signed-off-by: Lakshmi Shanmugam <lshanmug@in.ibm.com>