Single, unified Value node that picks its type from usage

This is blocked on having a way to pick the widgets for a node by clicking its expose dot, which will open a popover for picking the widget type:

Then the user can do this on the Value node (which is basically an Identity node generic over all types) which lets you pick the type to generate with the widget, and thus output with the single Value node.

Related: https://github.com/GraphiteEditor/Graphite/issues/2447

Start by reviewing related issue #2447 and the expose-dot widget-picker dependency described here. The work is complete when a single Value node can use the selected widget type to determine its generated value and output type.

Scritto dal modello di indicizzazione a partire dal testo della issue.