(svn r11882) -Codechange: introduce MarkSingleVehicleDirty() and simplify the code at some places

(svn r11881) -Cleanup: Code style for global variables.

(svn r11880) -Codechange: Generate vehicle group action dropdown dynamically, and assign an enum for the function return codes.

(svn r11879) -Fix: Only update group action drop down list when clicking on the dropdown text/button. Other times is not needed and caused a double free.

(svn r11878) -Fix: Crash in MP in vehicle group window if the currently selected group is deleted by another player.

(svn r11877) -Fix: another way to crash competitors' train in a station
-Codechange: increase the speed train is approaching line end/semaphore in non-diagonal direction a bit

(svn r11876) -Fix: Start industry tile animation also for overridden industry tiles. (Will only apply to new games or newly build industries.)

(svn r11875) -Fix: automatically sending aircraft to depot for autoreplace/renew is now triggered by the correct conditions
Now it triggers by the following conditions:
- aircraft is old enough for renew or needs to be autoreplaced
- player has 2*cost of new aircraft+autorenew money (we don't want to send many aircraft to hangars when there is only money for replacing one)
- aircraft has no hangars in it's order list
- new aircraft is buildable (imagine autorenewing a retired design)
This triggers right after a helicopter takes off or when a plane touches the ground while landing

Another effect of this change is that this functionality no longer generates network traffic

(svn r11874) -Fix [FS#1655]: all wagons of maglev/monorail trains would get the livery colour of the engine instead of their wagon type.

(svn r11873) -Codechange: less a few magical numbers and a tiny bit more comments on town zones