Enable loading styles from multiple import paths

Import was previously allowed from two locations:
* qrc:/QtQuick/VirtualKeyboard/content/styles/
* The first import path containing
  QtQuick/VirtualKeyboard/Styles/

This prevents an application-supplied style outside of
these locations from being used, even if the style name is unique.

[ChangeLog] Added support for styles in any Qml import path
under the directory QtQuick/VirtualKeyboard/Styles/<style>

Change-Id: Ie6034709182f182efacb0317e49d095d51ced5a6
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>