When editor continuous redraws is switched off, the editor only redraws when a redraw_request was issued by an element in the scene. This works well in most situations, but when scenes have dynamic content they will continuously issue redraw_requests.
This can be fine on high power desktops but can be an annoyance on lower power machines.
This PR splits redraw requests into high and low priority requests, defaulting to high priority. Requests due to e.g. shaders using TIME are assigned low priority.
An extra editor setting is used to record the user preference and an extra option is added to the editor spinner menu, to allow the user to select between 3 modes:
* Continuous
* Update all changes
* Update vital changes
a0c6d16c90
