Make the special character dialog extendable

[Wiktor Walc]

Right now if someone wants to add special characters to the dialog (like alpha, beta, gamma, delta), he is forced to change the source code of the editor.

In the simplest scenario we could at least have a configuration option, which would be used when constructing the chars array (a list of chars to be added to the end of the default array, alternatively the whole array could be a configuration option).

Sample post where user is asking for this feature: ​http://cksource.com/forums/viewtopic.php?f=11&t=18519

[Tobiasz Cudnik]

This feature (and related) are already pending review in #5599.

Resolving as DUP of #5599.