Add frames/time conversion functions to QML "filter" context object.

This will be used by the TimeSpinner.
Add TimeSpinner to Shotcut.Controls QML module.
Add Fade From Black video filter.
Add fade-in to timeline, works with Fade From Black filter.
Add audio Fade In filter with timeline fader control.
Fix fade in filters to be relative to in point.
Fix regression on filters without duration.
Add movit.overlay Fade From Black filter.
Reflect fade in control drag changes in the filter UI.
Fix height of timeline playhead cursor.
Add Fade To Black video filter (non-GPU).
Fix out point when adding filter.
Add audio Fade Out filter.
Fix the starting gain property on audio Fade In filter.
Rename audio fade-in filter's folder.
Add movit.opacity Fade To Black filter.
Fix trim in affecting fade-out and trim out affecting fade-in.
Remove unused code.
Fix fade-in filters relative to in point when initially added.
Fix typo in fadeOutMovit.