Sequencer: Tweak to how/when the "use_framerate" option is enabled

In response to feedback from plasmasolutions that having this option "always on"
caused some examples of more destructive errors, this commit introduces an attempt
at seeking a better balance between doing what people expect when casually importing
clips, and not clobbering existing timelines.

Specifically:
1) When there's nothing in the scene, the option will default to being enabled.
   This accounts for users doing this for the first time with no strips
2) When there are already sequence strips, the option will default to being disabled.
   In this case, changing the framerate could be dangerous. Hopefully this new check
   is sufficient.
3) There's always still the option for users to control what exactly happens. However,
   there have been reports that this cannot always be seen/set?

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