app, extensions, libgimp, plug-ins: Fix inconsistent action buttons position when is no header bar

The action buttons in some dialogs, especially the ones who use the new GimpProcedureDialog API, assumes a very weird (non compliant) position when is no header bar (eg Windows). And almost all the other GIMP dialogs (eg GEGL dialogs) don't use this position, nor do the OS guidelines recommend it.

actual position	expected position (Windows)

Now I know that this is due to the intent of GTK titlebar rendering (header bar). But, as Windows, for example, doesn't use it, this causes a wrong order.

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So this MR changes the behavior according to specific guidelines of each OS for better quality of life:

OS	buttons order
On Linux (remains the same):	[Cancel] … space … [Reset] [Export]
On Windows:	… space … [Reset] [Export] [Cancel]
On macOS (remains the same):	… space … [Cancel] [Reset] [Export]