Allow sheets to use symbolic parameters

[tomcl]

See extended discussion in #157

Design sheets would be a good way to implement more complex logic blocks - but for this to work they need parameters.

Something like the following:

1. Every integer in a component creation dialog can optionally be bound to a defined set of allowed alphabetic word parameters, in addition to an integer.
2. The list of allowed parameters is viewable and editable in a GUI popup form associated with any sheet and accessed via a menu item.
3. When a custom component is instantiated the GUI to do this requires values given to all parameters: these are then stored in the custom component instance.
4. A custom component's parameters can be instantiated as parameters in the sheet it is instantiated on - thus parameters can be passed through multiple levels of subsheets.
5. To keep things simple initially, expressions of parameters (n+1) are not allowed.
6. To allow configuration via parameters constants (most likely 1 bit wide) can be used to turn parameters into values in digital busses.

[tomcl]

see also #131 which this could solve with addition of:

7. parameters can enable or disable I/Os.