Add wasPlaying check to Playhead_Changed handler

Checks to see if Engine was previously playing before replaying it again.
Add Time/XCoordinate conversion method

Also adds XCaptionOrigin property which gets the X origin for captions.
Refactor: Move halfTimeWidth to private fields

Moves halfTimeWidth to the #region that contains private fields. Also
adds documentation for selectedCaption.
Add documentation to CaptionTimestampChangedEvent
Add Move Caption functionality to Timeline

Clicking on a caption will move it to where the mouse pointer is being
moved to. The caption's duration will remain the same.

Fixes error in XCoordinateToTime where method would not use its given
paramater
Add OnCaptionMoved Event to Timeline

Adds the OnCaptionMoved event to timeline, which is raised when a
caption is moved on the timeline by the user. Also adds handling of the
event on MainForm, which will update CaptionView
Add Video PlayheadTime label to Mainform

Adds a label that shows both the total length of the video, as well as
the the time that the playhead is displayed at.