Add seekbar, duration, and enable/disable toggle button

[AksharDP]

Added seekbar, current duration and max duration, toggle seekbar button in settings.

• Added SeekBarEnabled in UserSettings.cs with a default of True.
• Updated Settings.xaml to include a toggle for enabling/disabling the seekbar.
• Updated Settings.xaml.cs to handle seek bar toggle button in Settings.xaml.
• Changed MainWindow.xaml to create space below the thumbnail and media buttons for seekbar.
• Changed MainWindows.xaml.cs
  • Added timer to update slider every 300 milliseconds because MediaManager_OnAnyTimelinePropertyChanged only updates every ~5 seconds when the media application is not focused (maybe I missed something?).
  • Stops timer from running when Main Window isn't active to stop unnecessary updates.

Should close #17 if everything looks good.