clients: Allow transients to be minimized

Bug 14953

Until now, xfwm4 would prevent any sort of transients to be minimized.

Allow regular transients windows to be minimized, while still preventing
dialogs to be minimized independently from their parent window.

Note: This requires a fix in xfce4-panel taskbar to show those windows
(bug 15307), otherwise the user may not be able to restore the windows
once hidden.