Good news everyone! I'm just gonna share the workaround of the buttons problem that I've come with.
Just use the "Window Options" extension: https://extensions.gnome.org/extension/353/window-options/
There's only one problem – officially it doesn't support newer versions of Gnome(the last supported Gnome version is 3.8). But there's an easy way to make it work in Gnome 3.16+(tested on 3.16 but i'm pretty sure it will work just fine in 3.18 or 3.19 or some newer version in the future). All you have to do is download the extension manually, unzip it and modify the file
metadata.json with simply adding your version of Gnome in the list like this:
//… some text here … "shell-version": [ "3.4", "3.6", "3.8", "3.16" ], // … something here …
After that copy the entire
"email@example.com" directory to your gnome extensions folder. In my case (Ubuntu Gnome 15.10) this is:
and restart the gnome shell:
Alt+F2 -> r
now you just have to turn this on in Gnome Tweak Tool and your're done
P.S. I must say that Opera is the only software of what I use that doesn't have that window control buttons(minimize/maximize) so its really confusing when the devs say its Gnome's fault when it's really not because all other programs have this buttons.