Bug 1023087 - [Window Management] Marionette test for copy-paste

https://bugzilla.mozilla.org/show_bug.cgi?id=1023087
```
fakeTextSelectionApp.cut('centerInput', 0, 5);
assert.equal(fakeTextSelectionApp.centerInput.content, 'content');
textselection.paste('bottomRightInput');
assert.equal(fakeTextSelectionApp.bottomRightInput.content, 'text');
```