Advanced framegen logic

[yammes08]

Suggestion: add more advanced framegen logic:

• allow a user to manually trim a front and end % from the file to be screencapped
• Use (total frames) * (front trim %) as the start frame
• Use (total frames) * (end trim %) as the last frame
• Use ((total frames) - (front trimmed frames) - (end trimmed frames)) divided by (number of screenshots) as the frame interval
• Use a random offset to ensure mostly unique frames are generated
• store selected frame numbers for elsewhere in the application