Changed entry point to only initialize WinForms application when a WinForms message loop will be run. On Mac, the ExceptionDialog is prevented from appearing since it won't work, but this means errors checks regarding missing fonts can be removed since we won't attempt to show any WinForms stuff on Mac.
Fix some misapplications of String.Format and CultureInfo.CurrentCulture.
Fix GifDecoder and ImageSize to check for GIF version complying with standard format (two digit year and letter for revision, e.g. the current known versions 87a and 89a) rather than explicitly searching for 87a and 89a.
Fixed several problems contributing to a memory issue where the pony animator and viewer would remain in memory after returning to the menu, keeping memory usage unintentionally high.

- PonyAnimator will now remove event handlers that it attached to expireable sprites, since those would keep the anmator alive.
- MainForm and MainWindow will now ensure that global and local variables containing the animator and viewer are unset.
- MainForm and MainWindow will call for a full garbage collection to reclaim memory. This is done from the threadpool, to allow the AnimationFinished handler to complete and thus allow the animator to be collected.
- Shutdown of WinFormSpriteInterface will now wait for the UI to close and dispose before raising the InterfaceClosed event, so that the receiver can then be sure a GC would be effective.
- WinFormContextMenu and WinFormContextMenuItem will break references to work around the fact the classes they inherit from do not dispose properly and are kept alive for finalization. Breaking references prevents the WinFormSpriteInterface being kept alive whilst the menus await another collection run.
Several artwork tweaks by Bot-chan:

- Replaced idle image for Candy Mane with an animated gif.
- Replaced idle image for Dinky Hooves with an animated gif. Also, made a slight change to the eyes and tail to better match the trot sprites.
- Modified Bon-Bon's idle (mostly the timing) to look less fidgety. Small color changes (due to my programs first messing up the colors and then me actually liking the result).
- Animated the Tardis for both Doctor Whooves characters. Removed the hourglass animation from the fan-character due to his different design. Also made the Tardis appear less frequently for the canon character.