Disable client side decorations (GTK_CSD) by default on Windows (win32)

Submitted by Eduard Braun

Description

On Windows GTK3 windows look like aliens because they currently use client side decorations by default.

Even with the "win32" theme (which tries to emulate something Windows-7'ish but does not have transparent borders) it won't even blend in on Windows 7, let alone any other version of Windows.

Therefore I strongly suggest to disable GTK_CSD by deault on Windows which already makes applications using the Adwaita theme look more native than the "win32" theme does.

Version: 3.22.x