NSView+CGDrawing: make vertical gradient only apply in rect (bugfix)

- Forgot to clip to rect before drawing the gradient
Completely rework the custom window+titlebar

- Instead of adding the titlebar view to the theme view, add it to the
  content view instead
- Make titlebar view transparent (no drawRect anymore) so we can show
  the native system titlebar underneath
- Expand content view so that it fills the entire window
- Move library/playlist in content view to leave gap for titlebar 
- Use a textured background window so that the system titlebar style
  is drawn across the entirety of the window

- This guarantees that the titlebar matches the system, as it is the
  system drawn one
- This avoids the need for the hack in commit cbba0b8 (issue #47), as we
  are no longer adding any subviews to the theme view
- Also no longer using OSX 10.10 SDK functions, so code is the same for
  <10.10 and >=10.10

Thanks to Chrome's chrome/browser/ui/cocoa/version_independent_window.h
for ideas and hints on how to do this.
https://chromium.googlesource.com/chromium/src.git/+/30f5c966a41ba31b4d11d1804cf8d803ef1f0cc7
Allow window to be minimized