Resolution slider

You mean a spinner where you can enter the desired aspect ratio ?

For instance, you can do it, using this method :
- type in the desired width and height, and click on \"lock\" button.
- play with the slider, your aspect ratio will be preserved

Ouch !
That's an hard request...
Is this point really important to your eyes ?
changing this is really time consuming... it seems simple, but quite hard to develop, due to interactions with then render panel.
but ! i'll do my best...