shared drive as workspace directory

Currently we go through all the bundles for Css Theme contributions.
Instead it would be better if we get the contributions to the Extention
point "org.eclipse.e4.ui.css.swt.theme" and get the plugins that these
contributions belong to and only search in these plugins for the theme
related information. This patch implements the above idea. Also added
code to consider "org.eclipse.ui.themes" extension contributions.

Change-Id: I5ec7465b5d1717db1bf0779835f20ae01bacde6f
Signed-off-by: Kalyan Prasad Tatavarthi <kalyan_prasad@in.ibm.com>