gtk2: do not make floating windows minimizable

First, this feature leads to an horrible behavior on Windows where the application ends in a loop of endless minimization and restoration, probably because the window manager minimizes or restores the whole application once a floating window in minimized or restored, leading to a race condition between all floating windows, some being minimized and others being restored at the same time, triggering the minimization and the restoration of the others, and so on. It's difficult to say such issue will never happen on other OS or with some window manager.

Second, those floating windows are expected to be displayed or hidden using menu or keyboard shortcut, it's a design choice. Then the OS-specific way to minimize/restore them is superflous and less efficient.

Finally, the mainframe is not created as a floating window so the user minimizes the application by minimizing the mainframe.