Expose Particles PerfLevel

Qt Quick particles QQuickImageParticle has currently 6 different PerfLevels, which define the used vertex data and shaders. Proper level is selected based on features and the rendering backend. Lower the level, less memory usage and better the performance is.

Current PerfLevel could be exposed as an enum int QML property for the users.

• This would assist users to make optimal features vs. performance decisions.
• Document different levels. Documentation should state that this property should be used only for debugging and no promise is given that output remains the same in future Qt versions.
• Design Studio could show the PerfLevel for users automatically.
• Autotests could check the level to make sure that most optimal is selected with different features, and there are no regressions in this.