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.
Refactor Timeline.OnMouseDown method

Timeline.OnMouseDown no longer has a xPos variable, and instead now
uses e.X to refer to the mouse click point.
Merge branch 'master' of github.com:IMDC/EnACT
Additional Refactor of Timeline.OnMouseDown

Renames currentTime to mouseClickTime, adds comments, and makes beginX
and endX use TimeToXCoordinate.
Refactor Timeline.OnMouseMove

Removes xPos from OnMouseMove and makes mouseClickTime get its info
from TimeToXCoordinate
Add bounds check for setting Timeline Playhead

Setting the Timeline Playhead either by clicking or dragging will not
set the playhead to a time less than 0 seconds.

Removes xPos from OnMouseClick.
Add some comments to Timeline
Create new class: TimelineMouseSelection

Adds a new class named TimelineMouseSelection which holds the user's
selected caption or other item.

Moves TimelineMouseAction into TimelineMouseSelection.cs
Refactor Timeline.OnMouseMove: switch

Changes OnMouseMove to use a switch instead of if/else statements