Copy selected query plan representation

[antonkovalenko]

Currently when I click copy control in Query Explan Plan mode json representation is copied.

At the same time In computation graph mode there's no copy control. In AST and JSON plan modes it copies AST and JSON respectively.

As an application developer I want this control to copy Text repesentation when I am in Explan mode where text representation of a query plan is displayed.

[antonkovalenko]

in AST and JSON results received from back are copied
in text representation results are json.
so it is required to represent json into markdown