gtk: Patch to subtract _gdk_offset_{x, y} from the position of a child dialog.

_gdk_offset_{x, y} are the difference between the top-left of the primary monitor and the top-left of the entire desktop. Since WinAPI deals entirely with coordinates based on the top-left of the primary monitor, GTK adds the offsets to the x and y of a window when it's created via CreateWindow(). These values are then used as a reference for calculating the x and y of child windows via SetWindowPos(). Therefore, the offsets should be subtracted from the calculated position (say, center of the parent window) before calling SetWindowPos(). This patch does that.

Without this, the coordinates will have been calculated from the top-left of the entire desktop, but will be interpeted by WinAPI as relative to the top-left of the primary monitor, causing problems when the primary monitor's top-left is not the same as the entire desktop's top-left.

Fixes hexchat/hexchat#1007