cylc gui: add insert option to graph r-click menu

Relatively minor enhancement to the cylc gui arising from combination of personal irritation and user confusion:

There are times when a task is still visible in the graph view in the cylc gui but is no longer an active task. Occasionally users want to re-run an earlier task such as these. They can either run cylc insert from the command line or via the control menu. This pull request adds a convenience entry in the right-click menu for such (non-live) tasks which, when clicked, opens the insert task popup window with values for name, tag and family pre-entered.