Getters for copy information

[alschmiedt]

Is your feature request related to a problem? Please describe.
In the keyboard navigation demo I override the default paste functionality so I can paste the copied block to the current location of the cursor. In order to do this I need to use know what block was copied and from what workspace. This requires using Blockly.clipboardSource_ and Blockly.clipboardXml_.

Describe the solution you'd like
Getters for these fields.

Describe alternatives you've considered
Making the fields public. I'm not sure this is necessary. Since we have Blockly.copy, I don't see a reason why someone would want to set these fields manually.

Additional context

[George-the-1st]

I should be mentioning that I'm almost finished with this issue.

EDIT: finished and ready for merging. (done while ago but never updated here).

[cpcallen]

Closed by #5237.