UI: Highlight references to variable

[frothga]

Similar to a modern IDE such as Eclipse, when a given variable is selected, highlight all references to it in the same equation set.

Could create a background thread to scan the rest of the nodes in the tree and find the variable by simple string matching. The substring is very likely to be a reference if the character before and after it are not valid identifier characters. Or, we could do a limited amount of semantic matching by trying to resolve the candidate substring.

This highlight information could be attached to each tree node, and passed on to the the tree cell renderer to attach to the relevant text fields. When selection changes, this thread could be killed and replaced by a new one, or else the same thread could receive a new target and change what it is doing. There should be some delay from when a variable is focused until its references are highlighted, perhaps 500ms or so. Highlight color should be different (and more muted) than for regular selection.

[frothga]

The initial implementation does not use a separate thread or delay time, because it is fast enough to keep up with user interaction.