Remove need for parentAdjust in ScreenEntity and add UIMenuDivider (with scissor fixed)

This is the same as my last [pull request][1] but I fixed the issue with the scissor rect clipping. It appears that there was a bug in the scissor rect clipping code that just happened to work with the way the UITextInput sized it's text container (textContainer would stay the size of the scroll view but it would scroll with the text and the clipping code just happened to move the textContainer clip rect to the top of the scroll view). Once I added my Rectangle::Clipped() method (that had it's own bug that's now fixed) the scroll view clipping broke. I didn't fix the textContainer clipping rectangle because that's more change than I want to take on right now. I just only set it to clip when it's a single-line text input.

Brief summary:
- No more parentAdjust parameter needed for any of ScreenEntity's onMouse*() methods.
- Added a horizontal line menu divider to UIMenu so there's no need for having "-------" items. Dividers are also neither selectable or clickable.
- hitTest now works with arbitrary hierarchies of transforms.
- Made more methods const.
- Fixed a bunch of warnings on OS X (now nearly warning-free on OS X).
- Fixed scissorRect clipping.

[1]: https://github.com/ivansafrin/Polycode/pull/341