Develop concept for handling concurrency

[rempel]

Concurrency is not handled correctly in promot. Visible problems are for the browser delete of treeNodes #123 Editor #83, #121, #120 Explorer gives exception in the eventqeue for paint events (no ticket yet).

Synchronization is used for the update functions (coded by martin) but all other code ignores it (which breaks synchronization by ignoring aquired locks), so a concept for handling concurrency (which functions, which mechanism, guidelines for access of synchronized data, ..) is needed.

It is necessary that everyone understands how to handle concurrency (because one un-synchronized access can break the correctly synchronized rest).