Future Qt compatibility

[jdpurcell]

1. Limit workaround for QTBUG-125380 which was fixed in 6.7.2.
2. qt/qtbase@9bbfdd6 made QIcon::fromTheme start returning icons on macOS. There was already code to mostly turn off the icons, but it only affected the window menus. If you close all windows and were left with just the application's global menu bar or whatever it's called, icons were showing there. This also causes a serious delay any time a window is opened or closed, since something about menu icons in macOS on Qt is really slow and they don't get cached properly (remember we worked around that in the past for the Open With / Recent submenus). It looks like a later change, qt/qtbase@d671e1a, will hide menu icons by default on macOS, but my commit will ensure things work as intended with Qt 6.7.0 - Qt 6.7.2 if someone compiles qView for those versions.

3. qt/qtbase@fd97a82 implements a "safe area margin" concept on macOS which breaks qView's full size content view feature. With Qt 6.8 beta, this results in the graphics view widget not being placed under the titlebar as intended, meaning the cool translucent effect with the image underneath the titlebar doesn't work, and when fitting the image, the calculation is broken leaving a gap at the top. My commit opts out of this "safe area margin" feature.

[jurplel]

Amazingly researched, documented, split into commits, as always. Reviewing is a joy