add support for a manual expression controllers window

[sean-e]

Create a window to host expression controller widgets.

• no need for end zones, midi only (no adc sources)
• create command to assign a physical pedal to controller X or to focused widget
• only for globally defined controllers (no patch local controllers)
• ability to define up to 16 global controllers
• include refire button with each widget
• keyboard bindings: home/end/up/down/left/right/page up/page down
• numeric entry/display
• normal globalExpr programmable limits (min, max)
• command to hide/display the window
• upon pedal assignment, pedal is not active until sweep to current value?