Open linked processes in metadata editor

[solth]

This pull request adds a new option to the structure tree context menu in the metadata editor to directly open linked parent or child processes in the metadata editor, either in the current browser tab/window, thus replacing the currently opened process in the editor, or in a new tab/window.

Before opening a linked process (either via this menu option or directly via URL) the system checks whether the process is currently "locked", e.g. opened by another user, and in that case shows a corresponding message:

This is a convenience function meant to make it easier to manage metadata of process hierarchies.