Availability metric value should match to avail widgets

[jvalkeal]

Now availability metric is handled as normal metric. At least add AvailIcon(or similar way to track) to possible widget set.

[jvalkeal]

Actually AvailIcon is default for avail metrics but if you change it to something else you cannot switch back.

[jvalkeal]

Moving to target to future release. This actually needs a bigger conceptual changes. We need to have enough information to include/exclude resources in registry. Easy if data is based on metadata from tree, but if layout is loaded from backend there's potentially too much information what should not be stored to layout.

Maybe internal store keeping metadata for resources. Just need to think if this store makes designer too unreliable or slow if big layout is loaded and store is empty.