The `cursor` property accepts a list of possible cursors, which behave
as a fallback: We use whichever cursor is the first available one. This
is a little complicated because initially, any remote images have not
loaded, so we need to use the fallback standard cursor, and then switch
to another when it loads.
So, ComputedValues stores a Vector of cursors, and then in EventHandler
we scan down that list until we find a cursor that's ready for use.
The spec defines cursors as being `<url>`, but allows for `<image>`
instead. That includes functions like `linear-gradient()`.
This commit implements image cursors in the Qt UI, but not AppKit.
LibWebView now knows how to launch RequestServer and ImageDecoderServer
without help from the UI, so let's move ownership of these services over
to LibWebView for de-duplication.
We will want to re-inform WebContent of the system visibility state when
we create a new process after a crash. This changes the IPC to just send
the enum value directly, instead of a boolean, so that we can just store
that enum value directly on the ViewImplementation class.
