Cleanup integration test

[JordanMartinez]

To summarize:

• Splits NavigationTests into their own separate files for easier management
• Creation of new test base (RichTextFXTestBase) in case one needs to use a different area flavor (e.g. GenericStyledArea) while still having access to OS-related things and position helper methods
• Prevent possible interference with other tests (Only set up context menu in ContextMenuTests, clear clipboard before tests run)
• Optimize tests' string creation via StringBuilder to only do so once
• Use more accurate way of determining caret position via getCharacterBoundsOnScreen
• Rename nested classes and test methods to use "_" as a space-like character for easier readability
• No longer use glass robot by default; CI builds will determine this now
• Moved some tests to more appropriate files with related tests
  (see commit messages for full list of changes)

[JordanMartinez]

I'm out of time for today, but I am going to see if the Mac build will pass if I run the HitTests tests that currently fail on Mac now that we have a more accurate way of determining the caret's paragraph

[JordanMartinez]

@JFormDesigner Per your comments here, I've rewritten some tests to use the getCharacterBoundsOnScreen API to more accurately test the caret's position.

[JordanMartinez]

AppVeyor is still not set up yet, but the tests so far pass on Travis and Mac (aside from the two bugs discovered: #602 and #603)

[JordanMartinez]

@afester I appreciate that you submitted the PR, but I decided to revert what you did in #589 (the glass adapter) in this PR. We may need to revert my change, so that it's back to yours (at least when Gradle detects that Windows is running it). I hope to know more once I get the Windows CI set up properly.