Added a export-to-PNG button in the dialog that is launched after invoking org.antlr.v4.runtime.misc.TestRig with the "-gui" parameter. When pressed, a PNG file of the parse tree is created in the present working directory.
Merge remote-tracking branch 'bkiers/png-export-tree'
Fix formatting problems in pull request
Move "png" button to the right of "OK", refactor export action to its own method