The tabOrder vector was initialized with the size of tabNames.
Then new tabs were registered with AddSetupTabCallback() causing
tabNames to grow but tabOrder to remain the same.
Usually this is not a problem as LoadTabOrder() will be called which
ensures that their sizes are in sync.
However, when using a fresh OBS install or one with a corrupted scene
configuration file, the on_save() callbacks are called before any
loading callbacks.
With the sizes of tabNames and tabOrder not being in sync this would
lead to a crash.
Fixed by adding a tabWidgetOrderValid() check within SaveTabOrder()
before iterating over both vectors.
The tabWidgetOrderValid() was expanded to check the sizes of both
vectors.
Additionally the tabOrder initialization was moved from global static
initialization to function local static initialization to ensure that
potential preceeding calls to AddSetupTabCallback() will be taken into
account when setting up the tabOrder vector.
