Flow element local space tool

[nikku]

When changing existing processes users would often like to insert elements in a given flow, i.e. because they refined or restructured the process (cf. #875):

The current space tool is a global tool and as such not suitable to support the user.

Idea

When starting the move space operation on top of a flow element with a modifier pressed the tool becomes flow element aware and selects the element and all connected elements left of or right of it (depending on the space direction).

Tasks

• A local space tool mode moves an element + all connected elements left of it or right of it (depending on the space direction)
• The local space tool mode activates, if a user initiates the space tool on top of an element, with a modifier pressed
• Connected elements = elements in the same BPMN scope, connected via bpmn:Association or bpmn:SequenceFlow

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Related to #178