Bigger or smaller Font Sizes based on the platform default.

Also relates to task 153335,

The different platforms have different pt sizes for their default fonts, when creating a cross platform application the developer might want some text to be bigger or smaller than the default font - example (default + 1 or default - 1)

In Qt code the suggested method is to get access to the default font and then modify it within the application.

This is currently not possible in Designer, where absolute values need to be specified.

It would be nice if designer could be used to specify "a bit bigger (or smaller) than default" font sizes.