Editor: should show that the editor can be pushed in a non-intrusive way (alternative to notifications)

[adietish]

follows up on #763

In #763 we allow to disable push notifications that are displayed when the editor content is out-of-sync with the cluster (resource does not exist yet or is modified). The reasoning is that those notifications are a distraction. We could still show to the user when he can push to the cluster but do it in a less intrusive way: ex. by highlightling the push icon in the toolbar (similar to the "*" that some editors - ex. Eclipse - would display next to the editor title when the file is modified but not saved yet).