Show caller when asking for readout permission

Get stack and show the caller when asking for readout API permission.

Related to enhancement request to show source where canvas is nested.
https://github.com/kkapsner/CanvasBlocker/issues/8
Merge pull request #10 from jay/stack1

Show caller when asking for readout permission
Built with caller display.
Added preferences to display caller(s)
Merge from master