navigation through bookmarks only considers the current script

[M. Gronle]

The bookmark management of scripts is currently managed script-internal. Therefore the navigation buttons only consider all bookmarks in the current script. This should be changed to a global bookmark management.

[M. Gronle]

fixes issue #112: bookmarks are now managed by a central bookmark model. All bookmark backward / forward buttons are now deliverd by this model, as well as the delete action. Additionally, there is now a bookmark toolbox, similar to the breakpoint toolbox, that allows jumping to different bookmarks and navigating through the list of bookmarks.

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