Inventory system added

Added a basic inventory system that uses the item's name as a key.

Players can obtain items, check how much of a type of item he possesses,
and lose items.
Found a sprite sheet with most of the required animations.
Finished the player character sprite sheet

Adjusted the existing 8 directions of flying animation, and drew the 8
directions of walking and idling.  Implemented the sprites to fully
animate the player character.  Some fine-tuning is needed, but this is a
major milestone nevertheless.
Merge animation branch
Add quest system prototype
Display dialog when hover on sign
Changed diamonds.png
Some more images prepared

Items and Prof Oak(BW)
Changed signpost to professor oak

...I don't think this needs to be animated though?
Also changed x from 100 to 95 for the sprite to fit in the middle of the
tile
UFO.gif sprite

Possibly usable for enemy
Added dynamic drawing order

Drawing order now changes based on the following priorities: entity
height, then entity y position.  Based on the drawing order, entities
are sorted via insertion sort.
Fine-tuned player animations, added speed adjustments

The player character now has more "realistic" animations, as defined on
the coding side.  Speed can now be adjusted by holding down Q or A to
speed up or slow down respectively.  This also affects turning rate.
Speed is now calculated using a psuedo wind drag formula.
Rough work
Positioning that can work with longer sentences
refractoring
Implemented island collision detection.

The player should no longer be able to walk off islands or land on sky.
The debug tile highlight does not work perfectly when the player is
turning, but it should work fine if the coordinates are updated at least
once per frame.
Static background

Changed the sky background to be static instead of being at the top left
corner of the global map.
Changing styles of that dialog frame

Finally, it's that rounded-edge rectangle (not patented by Apple,
hopefully)
Naming changes
Styling