editbox lacks a single listener for all text changes

[gajop]

Text in editbox can be changed by at least three different ways:

1. Manually setting it with SetText - which doesn't invoke a listener
2. Text deletion, copy, paste - which invokes an OnKeyPress
3. Unicode text input - which invokes OnTextInput

OnKeyPress and OnTextInput can also be invoked in some cases where no text has been changed. I suggest we add a new, "OnTextChanged" event that will be invoked for all changes of text, otherwise we'll have ugly code: https://github.com/gajop/Scenario-Editor-Core/blob/master/scen_edit/view/unit_defs_view.lua#L4-L21