Adding GetMinTime & GetMinFrame methods to Timeline

[jonoomph]

These are used to get the start time and position of the first element on the timeline. This will be a new UI function in the next version of OpenShot, so an export can quickly trim the start or end to the clips/effects on the timeline.

[jonoomph]

Dragging playhead, clips, or transitions past the start of the timeline, should stick to 0.0 (not jump back)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.42%. Comparing base (118810f) to head (959947a).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #974      +/-   ##
===========================================
+ Coverage    54.24%   54.42%   +0.17%     
===========================================
  Files          182      182              
  Lines        16513    16578      +65     
===========================================
+ Hits          8958     9022      +64     
- Misses        7555     7556       +1     

Flag	Coverage Δ
	54.42% <100.00%> (+0.17%)	⬆️

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