On Windows:
Meta+F1 = open system help
Meta+Tab = cycle through taskbar buttons
Meta+F = find file

even more:

Ctrl+Meta+F = find computer
Meta+D = display desktop
Meta+E = open file manage
Meta+L = lock desktop
Meta+M = minimize all windows
Shift+Meta+M = undo minimization
Meta+R = open command window

OK, I added a section of explanations on how to avoid
keyboard clashes with existing shortcuts.

As to Alt-based shortcuts, please do the necessary so that
those ones which clash with the system shortcuts will not be
defined as shortcuts in the menus. I already did this for
the documentation: see the routine kbd_system_prevails
in tm_config.cpp for the criterion that I have used.
Notice that the Escape key will be equivalent to Alt
for such cases.

Is there at least a way to detect which shortcuts have
been reserved by the OS, so that we can avoid them?

It seems that some keyboard shortcuts (Ctrl+1 inside spaces
on MacOS) are not redefinable for specific applications.
Is there a workaround?

For Alt-based shortcuts, it suffices to define them
as a shortcut inside Qt. At least this seems to work for
most of them.

Another question is which ones we consider so important that
we indeed want to override the system settings.