Provide a better scaling system

At the moment, scaling is quite bad.
We only change number of columns and rows and -since Slingshot requires at least 4 columns and 2 rows- we only change a little.
This means that if you run Slingshot on a 800x600 screen you get only 8 huge apps per page, while it'd be better to scale icon size as well.

While rethinking the scaling system, we should take into consideration those who have manually changed options. If they've changed to be greater than 2/3 of the screen size, why should we scale them to 2/3 when resolution change? It'd be better if we record the per cent of screen used (on both x and y) when options are changed and keep those proportion regardless of screen size.