Make it easier to minimize windows

Several users asked to be able to minimize windows, gedit for example.

This is possible, with a right-click on the title-bar, but this is not straight-forward for many user.
We may want to reintroduce the buttons

For the record:
gsettings set org.gnome.desktop.wm.preferences button-layout appmenu:minimize,maximize,close

Original created by @elouann on 11367 (Redmine)

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