Fix #7847: Use ViewportSign coordinates for sign Kdtree coordinates (#7849)

Ensure the same coordinates are used for station/town/player signs regardless of how the landscape changes below it after the coordinates were first determined.

By keeping track of whether each ViewportSign is valid for Kdtree use (and only ever registering the viewport sign when the object is valid) a lot of code can be simplified and become more robust at the same time.

(svn r15027) -Merge: tomatos and bananas left to be, here is NoAI for all to see.
NoAI is an API (a framework) to build your own AIs in. See:
http://wiki.openttd.org/wiki/index.php/AI:Main_Page
With many thanks to:
- glx and Rubidium for their syncing, feedback and hard work
- Yexo for his feedback, patches, and AIs which tested the system very deep
- Morloth for his feedback and patches
- TJIP for hosting a challenge which kept NoAI on track
- All AI authors for testing our AI API, and all other people who helped in one way or another
-Remove: all old AIs and their cheats/hacks

(svn r14345) -Fix: delete the RenameSignWindow when 'its' sign is deleted
Also, it makes sure the RenameSignWindow isn't open when there are no signs (and crashes associted with that)

(svn r11822) -Codechange: Replaced fixed size custom name array. Names are now attached to their object directly and there is
no limit to the amount of names.
-Fix: NewGRF engines could not be renamed.

(svn r9884) -Feature: 2 new zoom-out levels: 8 times and 16 times
-Codechange: unified the blitter function so we have 1 function for all zoom-levels
-Codechange: make most of the label functions work with zoom-level instead of magic numbers and big switches per zoom-level
-Codechange: MakeXXXDirty() functions didn't take into account zoom-level, but just used the biggest possible value
-Codechange: simplified blitter functions to avoid code duplication