titled-dialog: Show min and max buttons (Fixes #26)

Since GtkDialogs with CSD only have a close button and our settings
dialogs are only "dialogs" we want to show the min and max buttons too
there. This means we have to set the type-hint for the window manager to
"normal" and to be sure we also manually center the window on the active
screen.
3 jobs for !20 with issue-26 in 2 minutes and 9 seconds (queued for 2 seconds)
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