Fix --full-screen option from export window

[sampottinger]

Fixes #488. Clean up the footer writer section of the preprocessor for easier reading but also fix adding and looking for --full-screen which appears to have been left off at some point. Specifically:

• Though not the source of the issue, the footer writer section of the PdeParseTreeListener was a little hard to read so cleaned up that code slightly.
• Found the issue with the handling of --full-screen which required a change in PApplet
• Added tests for the full screen export option for the parser / preprocessing

[benfry]

Thanks!