Add support for finding toolbox elements

[glopesdev]

This PR adds basic support for finding toolbox elements in the workflow. The current implementation works much like the "Find Next" and "Find Previous" logic common to text editors, where the workflow view will highlight the subsequent match in topological order. Search is initiated from the toolbox with support for three different toolbox element types:

• Subject: If a subject node is selected, all references (subscribe or multicast) matching the selected name will be found.
• Workflow: If a workflow node is selected, all IncludeWorkflow operators matching the selected workflow name will be found.
• Other: If any other node is selected, all workflow operators matching the type of the selected toolbox node will be found.

The selected shortcut key is F3 for "Find Next" and Shift+F3 for "Find Previous", following text editor convention.

Fixes #701