rcp: Fix icons for project explorer on Eclipse 4.6M5

rcp: Fix icons for project explorer on Eclipse 4.6M5

All the platform icons where changed to png file types and file
extension. Since in the RCP we need to register these images for
the RCP product and there are no constants available we need to check
for gif and png file extension to support Eclipse 4.5 and 4.6.

Change-Id: I2d751df1177adc991e300ec9e12a19922c163fe9
Signed-off-by: Bernd Hufmann <Bernd.Hufmann@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/65505
Reviewed-by: Hudson CI
Reviewed-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>