Move menu interfaces to separate package

Change-Id: I82ecfe59be2cd65ad65da8ab4bf23b5bcbb0f96f
Signed-off-by: Denis Knoepfle <denis.knoepfle@sap.com>
Refactor ExtensionItem and handlers

Refactored extension item to be able to add new functionality depending
on interfaces IDeletable etc. Prepared for changes in the way handlers
are accessed when processing selections in the commands. This is useful
for easier reuse in context menus later.

Change-Id: I579e461e672677be7c71e016a83fd05bfdfdaba7
Signed-off-by: Denis Knoepfle <denis.knoepfle@sap.com>
Refactor navigator classes

Adjusted navigator classes to new custom handler framework.

Change-Id: I7c34ca22b938330de5dfc6c858fab1f0f0503b6b
Signed-off-by: Denis Knoepfle <denis.knoepfle@sap.com>
Merge pull request #136 from DenisKnoepfle/master

Big refactoring of ExtensionItem and custom handlers