Rationalize image searching and caching

The current image infrastructure has a number of different caches for images:

- the `Image` trait caches `str -> ImageResource` lookups: https://github.com/enthought/pyface/blob/2dae9b473ea05df4917919582f52c590e4d1dceb/pyface/ui_traits.py#L57
- the `convert_bitmap` function in `pyface.ui_traits` also creates a cache of `ImageResource -> bitmap` lookups, but does not specify sizes, so gets whatever the natural size of the image is: https://github.com/enthought/pyface/blob/2dae9b473ea05df4917919582f52c590e4d1dceb/pyface/ui_traits.py#L81
- there is the `IImageCache` class used by `Action` instances to cache `filename -> image/bitmap` lookups for images of a particular size: https://github.com/enthought/pyface/blob/2dae9b473ea05df4917919582f52c590e4d1dceb/pyface/i_image_cache.py#L17-L70

There may well be others.

There is a certain amount of inefficiency in all of this:

- `ImageCache` instance are unique for each `ToolbarManager` and so are only caching images for a particular toolbar widget; in particular different windows with the same toolbar structure won't share caches, even though they may have all the same images.
- To create an image for the `ImageCache`, code currently calls `ImageResource.create_image`, ignores the result but grabs the _path_ which is now populated, and passes that to the `ImageCache` which re-loads the image and scales it, despite the fact that `ImageResource` has code to search for similar images of different sizes (using "images/WxH" subdirectories).
- worse, `ImageResource` stores a private `_ref`, which is a reference to an image file, but sizes may have multiple files, so the `_ref` is only good for one size (I think the last requested), so the absolute path used by the ImageCache is not even unique.

This doesn't even consider `ImageLibrary` instances.

This all needs to be simplified and rationalized and made so that it actually caches effectively.