app: fix max size computation for status message.
Our error/message handling code was checking if the status bar label was big enough to hold the message by checking the GtkLabel allocation. This means that error message ended up on status bar only if a status text bigger than the error message was previously displayed. Even setting gtk_widget_set_hexpand() or the "expand" container child property on the label, I could not find a way for GTK to actually give it as much space as possible on the status bar. Instead, I am computing the full container box size, starting from the label x coordinate (assuming the label is the last shown widget on the status bar, as usually the progress bar and the cancel buttons are not shown in the same time as the message label). This gives me a much more appropriate result of the maximum size which this label can hold without ellipsizing.
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