gtk3: gtk_widget_register_window ist available only in 3.8

[Friedrich Beckmann]

Hi,

i did some changes in the gtk3 branch and tested on debian 8 (current stable release). 

I used gtk_widget_register_window() which was introduced in gtk version 3.8. This 

does not compile on debian 7 because there we have version 3.4.2. I can change

the required version check in the build process or replace it with the previous construct.

Any opinions regarding a change of the minimum required gtk3 version?

Friedrich