New port: x11-toolkits/qt5-gtkplatform

gtkplatform is a Qt Platform Abstraction plugin providing Qt applications with
the capability to use GTK+ as a host toolkit.

It lets Qt applications render with native GTK+ menus, and use GTK+ for input
(mouse, keyboard, touch), and getting window content on screen, the same as it
uses e.g. Cocoa on macOS.

WWW: https://github.com/CrimsonAS/gtkplatform

PR: 224453
Submitted by: Greg V <greg@unrelenting.technology>