Turbo fix

[SuuperW]

I decided to go with the option of turning TAStudio's _seekingTo into a property backed by MainForm.PauseOnFrame. I avoided the issue of using MainForm's end-of-seek logic by having it update tools at the end of a turbo seek, in the normal place (which happens before end-of-seek logic) instead of in the end-of-seek logic. Cancelling a seek by setting PauseOnFrame to null no longer updates tools, because (a) this isn't needed anymore when a seek ends; and (b) that logic was unable to have the core render a frame, so it only did part of what needs to happen when a seek is cancelled.

In order to properly cancel a turbo seek, we need to emulate one more frame. So TAStudio's StopSeeking has additional logic for that now.

Clicking the icon at the bottom of the main form to "stop seeking" does not actually set PauseOnFrame, it just pauses. So issues still exist there. I decided against fixing that because it's a separate issue and isn't trivial to fix. (setting PauseOnFrame there would break the pause at end of movie feature for regular movies)

Pausing during a turbo seek will not result in rendering or tools updating. This maybe should be changed, but that also is a separate issue.

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[Morilli]

I understand the intention behind this but it doesn't seem to really work when the emulator is paused. Canceling a seek will just look like it didn't work because the seek will only be finished on the next frame. And emulating a frame in response to a cancel seek request doesn't seem right either. It's probably easier to just not do this.