[UX] BPMN modeler variables tagging

For each process variable defined in a process. Users should be able to tag each variable with certain values. This allows tweaking the process runtime execution.
Possible values:
internal - make variable to not be exposed in the model and consider as process instance internal variable
required - requires to be given to start a process instance
readonly - can be set only once
input - marks given variable as an input of the process and by that not exposed in the returned data model
output - marks given variable as output and thus not expecting it on starting and will be included in the returned data model
business relevant