Add OnChange handler to the TextInput widget

[LQR471814]

This PR adds an onChange option to the TextInput widget. This is an event handler similar to the existing OnSubmit, it is called whenever a character is inserted or deleted from the TextInput's text content.

Rudimentary tests have been written and an example of it's usage is given in textinputdemo.go.

[mum4k]

One more request, can you please give me an explicit permission to merge this code into the master branch when releasing the next version? I will need that to bypass the CLA check at that point.

[LQR471814]

The requested changes have been made. I give you explicit permission to merge this code into the master branch and do whatever you want with it.

[mum4k]

I apologize for the delay, I did not notice the comments were already addressed. I will look at this in the next few days.