Recent update/commit: Support blitting with changing canvas in some widgets

Approach depends a bit on the widget, but generally:

• self._useblit only stores the flag passed to__init__
• canvas.supports_blit is checked dynamically when needed
• we sometimes have the propertysef.useblit, which is the effective
  value of flag and capability
• if animated artists are used, that state is set during
  __init__ based on the flag. This relies on the fact that
  the flag cannot be changed later, and that the value of
  "animated" does not play a role for drawing canvases without blit support.

Supported:

• Button
• _SelectorWidget
• SpanSelector
• ToolLineHandles
• ToolHandles
• RectangleSelector
• LassoSelector
• PolygonSelector

Not supported yet:

• CheckButtons
• RadioButtons
• Cursor
• MultiCursor
• Lasso

This should already fix#30575 as Selector widgets are covered.

Note: It's probably best to review the commits individually.

Drat, I did not re-look at this PR before I opened#30605

Rebased.

@tacaswell How do we want to continue here? I consider this PR complete for the handled widgets, and suggest that we review and merge it. The remaining widgets should be handled in a follow-up PR to keep this PR more manageable.

Here's a summary of this PR approach:

• First commit: Create an infrastructure that safely handles background images by storing them on the canvas and defining an API in a way that there is no guarantee a background image is available. Consumers can try to obtain a background but must gracefully handle the case that no background is available.
  This is the basis to be able to reasonably handle blit-backgrounds while swapping canvas. Apply this to widgets that store backgrounds.
• Second commit: Fix dynamic changes inuseblit on the individual Widgets case-by-case. Since logic slightly differs between widgets, we have to think them through individually. The list of currently fixed widgets is inFIX: Make widget blitting compatible with swapped canvas #30591 (comment).