Performance improvements and bugfixes.
-----------
The whole program should run a lot more faster with the threads on it.
Inventory.java, InventoryEntry.java:
	- Inventory system has been fixed, now every type saves and accepts Inventory.
	- Deprecation of some methods that can't be implemented. For those methods exist easier alternatives.
Main.java:
	- Rewrote drawing process.
PreWindow.java:
	- Adjusted class to threading system in Main.
ArrowSelection.java:
	- Adjusted class to threading system in Main.
	- Bypasses now Mechanics.java, else it would not work.
Screen.java:
	- Adjusted class to threading system.
Item.java:
	- Added method "getMaximumStackCount()" for inventory purposes
	- Renamed it back to "Item.java" from "Arrow.java". It would not make any sense with "AbstractArrow.java"
AbstractArrow.java:
	- Adjusted class to changes in superclass "Item"
Keys.java:
	- When pressing "Shift+Escape", program does not exit directly, leaving a possibility for cleanup.
Mechanics.java:
	- Optimized "roundTo25(...)" method

This commit adds a massive performance boost to the program flow by usage of threads.
Some unclearness should also be solved with this commit, because some methods
that could behave erratically have been removed or rewritten to work correctly now.
Merge branch 'master' of https://github.com/iApocalypsee/Pfeile