feat(engine): enable async start events

- a start event may be declared async such that on execution, a new
process instance is created, but no activity executed yet

related to CAM-424
test(engine): test execution listener invocation on async start

related to CAM-424
feat(engine): enable relative task due date

related to CAM-889
docs(engine): improve JavaDocs of TaskService and Task
feat(variable): Add variable to process instance

- Now it is possible to add a new variable to process instance.
- Therefore a dialog exists, which can be triggered via a button in the
  toolbar inside the process instance view.

- Implements a new directive called 'variable', which handles the input
  fields to add/edit a variable

- null variables can be added too now

related to CAM-1082, CAM-1115
feat(CA-welcome): improve get-started experience

improve getting started experience on prepackaged distribution. This
should include an already installed admin user so that a new user don't
have to click through the setup dialog in camunda admin. (we create a user
demo with his credentials via Invoice-example)

Closes: #CAM-1125
fix(webapps): allow cockpit plugins to work with authorization

Closes CAM-1089
Merge branch 'master' of github.com:camunda/camunda-bpm-platform
fix(welcome-page): remove unused and commented out alert box
Merge branch 'master' of github.com:camunda/camunda-bpm-platform
fix(tasks): Fix unnecessary deletion of tasks

related to CAM-1128
Merge branch 'master' of github.com:camunda/camunda-bpm-platform
chore(test): Add a test case

related to CAM-1128
feat(tasks): Show user task process instance view

*cockpit
- adds a new tab "User Tasks" in the process instance view in cockpit
- the tab contains the current user tasks of the corresponding user task

*engine/REST
- adds activityInstanceIdIn to the query
- adds some testcase for the query

related to CAM-1070
feat(admin/notifications): add post setup notification
impr(admin/users): usability improvements

- add confirm dialog to user delete action
- code cleanup
fix(webapps/notificationsPanel): fix panels not being removed
feat(variableType): Change type of a variable

- adds the possibility to change the type of a variable in

related to CAM-1130
impr(notifications): update position and naming

- notifications are now positioned in the top right corner of the screen
- update naming of notifications

Closes CAM-1055
Merge branch 'master' of github.com:camunda/camunda-bpm-platform