Add sorting for properties (issue #51)

Added viewer comparator to properties viewer. Elements are sorted
alphabetically now according to their displayed name, so changes to that
affect the order. Whether the sorter prioritizes mandatory-flagged
properties can be toggled via the name-column (click), by default it's
turned off.

Change-Id: I1768a0e02242fb1946e414bb563904c9731fb948
Signed-off-by: Denis Knoepfle <denis.knoepfle@sap.com>