Showing the tip will in turn call getDesktopScaleFactor(), accessing the
lastComponentUnderMouse. In some cases, it was possible for
lastComponentUnderMouse to point to a deleted component, resulting in
UB.

There are two changes in this PR:
- Using a SafePointer rather than a raw pointer ensures that calls to
  getDesktopScaleFactor() will always be safe, regardless of when they
  happen.
- Moving the assignment of lastComponentUnderMouse to before the call to
  displayTipInternal() ensures that the returned scale factor is that of
  the component that the mouse is currently hovering.

This commit removes the various compiler-specific JUCE_DEPRECATED macros and replaces them with C++14's deprecated attribute. It also removes the JUCE_CATCH_DEPRECATED_CODE_MISUSE flag as we can rely on the override specifier catching usage of these old virtual methods, and tidies up the DOXYGEN preprocessor checks as they were inconsistent across the codebase.