Multi field statusbar and usage hints

[nothingTVatYT]

The basics are there:

• a new MultiFieldStatusbar that is code-compatible to the Statusbar but can hold multiple distinct fields. The default is to the left and 60% of the remaining horizontal space is used for displaying usage hints. The progress bar and the resize handle are unaffected by this change.
• some usage hints for editor windows. I picked those I personally would have loved to know from the beginning and like to be reminded of.

I hope for feedback on what you think which input actions should be shown in the usage hints.