fix manual preview button

[ZigZagT]

This fixes a issue where the addon keeps generating previews on every change as soon as the "update preview once" button is tapped at least once

I believe this also fixes #121 although the issue did not include sufficient detail to pin point the root cause.

reproduce steps:

• open "generate bin" command
• make sure "show preview" option is off
• click "update preview once" button once
• make a change to other parameters (i.e. bin length)
• the preview now refreshes, which is unexpected

root cause analysys

I think this is because showPreviewManual.value = False did not work as intended. Although it's counter intuitive, it actually makes sense if you think about it. A change made in event handler would trigger another event and resulting in infinite loop. I think Fusion is smart enough to ignore such changes to avoid hanging up.

The fix

I implemented this fix with a global variable because I can't think of a better way. It works during my testing but I honestly don't know if this fix will introduce unexpected side effects. Suggestions are very welcomed.