Fix correct usage of ISelectionService and ISelectionProvider

Adjusted to Eclipse' regular handling of providing and accessing
selections within the workbench. This fix also prepares for easier reuse
of commands in context menu for the extensions viewer (preparations for
changes for #101).

Change-Id: If3c356633e8a9ef67bfae271a7380349819d2209
Signed-off-by: Denis Knoepfle <denis.knoepfle@sap.com>