added disableStyleCopyPaste property

[melchiar]

Added a new property disableStyleCopying to the text class, as discussed in #4981. Enabling this property essentially turns of the default Keep Source Formatting behavior of the existing copy paste functionality. Useful for applications where a single text style is desired for each text object.

@asturur, do you think we need any unit tests for this?

[asturur]

in test/unit/itext_key_behaviour.js line 266 we have a test for copy.
We can agument it ( or create a copy of it ) to test this new property.

[melchiar]

Added a property check here to prevent style pasting (in case text was copied before the property was changed). Is this an okay place to put it?

[asturur]

@melchiar i m merging this and adding a unit test for it.
I have been busy and i do not want drag you around for long.
Generally we want always to add one or more tests for the following reasons:

1. future contribution may break this feature
2. a test usually fix what is the intent of the code and in case of breakage explains you what you were expecting and how you should fix it.
3. peoples/companies can trust libraries with tests more.