Allow to define externs of type cplx within exprs.

[searles]

This is a part of preparation for on-screen editing of points.

• Use all extern cplx for on-screen editing [allow some selection].
• Mechanism to have two bitmap fragments next to each other and select, where one of the extern cplx should be edited (eg julia split)
• It should be possible to define externs within 'function'. Eg enter "newton(z, (z-p1)(z-p2)(z-p3))" as function and show a list below to add new externs.
  ** These externs should be added automatically based on 'function'
  *** Needs parsing information what variables exist in expr
  ** Only 'cplx' should be allowed?
  ** Need new extern type that packs 'externs' and 'expr'. What about 'function'?

[searles]

Most likely I need a function activity. Mock-up as follows:

Would actually be good to have a unified framework for all activity frameworks wrt copy/paste/save/load.

[searles]

This should be as easy as possible! If should be really just using a variable name that does not exist yet inside. For instance, z^3 + a * z^2 + b * z + d.

[searles]

With some nice feedback this could be quite good actually...