[ENH] signals: Explicit insert/remove handlers

[ales-erjavec]

Issue

Ref: biolab/orange-canvas-core#86

Requires biolab/orange-canvas-core#161 to be merged and released.

Description of changes

Add a new MultiInput description to managed explicitly ordered sequence of (multiple) inputs.

i.e.

...
class Inputs:
    foo = MultiInput(...)
...
@Inputs.foo
def set_foo(self, index, foo):
   self.foo[index] = foo

@Inputs.foo.insert
def insert_foo(self, index, foo):
    self.foo.insert(index, foo)

@Inputs.foo.remove
def remove_foo(self.index)
    self.foo.pop(index)

Includes

• Code changes
• Tests
• Documentation

[ales-erjavec]

Unfortunately this solution would not properly handle undo/redo in canvas: biolab/orange-canvas-core#118. It can only 'communicate' link appends not inserts.

[markotoplak]

The concept of the solution looks good to me.

@ales-erjavec, is this still RFC or could it be merged after canvas-core release?