#86 getting and updating stylesheets

[Phillip M. Feldman]

The following seems to be "missing" functionality in the Qt API:

(1) One can set a stylesheet of the widget, but there is no mechanism for getting the stylesheet.

(2) It would be useful to be able to update specific properties in an existing stylesheet without replacing the entire stylesheet.

[Uwe Rathmann]

Unrelated to the Qwt project.