Observed on Ubuntu Linux. Occasionally, the loop checking the
condition_variable in the plugin scanner would spin indefinitely.

The cause appears to be that handleMessageFromWorker could be
called immediately after sendMessageToWorker, but before locking the
mutex. If this happens, gotResponse will be false during every call to
condvar.wait_for, and the loop will never exit.

The rewritten version of the scanner always resets gotResult immediately
after the condvar is woken successfully, so a call to
handleMessageFromWorker or handleConnectionLost will always cause a
subsequent call to condvar.wait_for to exit successfully.

The Superprocess class has also been refactored and extracted to avoid
a circular dependency between Superprocess and CustomPluginScanner.

Since the window borders are unknown for a short time after window creation on
Linux, restoreWindowStateFromString() needs to adjust the restored position
before passing it on to the XWindow system.

The adjustment is dependent on whether we use the native or the JUCE titlebar.
Hence the adjustment can only be correct if the titlebar configuration has
already happened before calling restoreWindowStateFromString().

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.

- Add AlertWindow::show() and showAsync() methods that take a MessageBoxOptions argument
  - Add NativeMessageBox::show() and showAsync() methods that take a MessageBoxOptions argument
  - Update the DialogsDemo to demonstrate the new methods
  - Deprecate AlertWindow::showNativeDialogBox() in favour of the NativeMessageBox methods
  - Pass button strings specified in MesssageBoxOptions to native dialog boxes correctly
  - Use modern TaskDialog on Windows for the native dialog box where available

See BREAKING-CHANGES.txt for more details.

The increased border size should make it slightly less fiddly to tap on
the window border.