Codechange: Template DoCommandP to automagically reflect the parameters of the command proc.

When finished, this will allow each command handler to take individually
different parameters, obliviating the need for bit-packing.

Codechange: Let the compile generate the master command table out of templated command traits.

This is using a non-intrusive type-traits like templated system, which
allows compile-time validation that the command table and the command
enum match up.

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.

Fix #7410: Sign position/width not set on initial creation. (#7413)

Sign width was only updated when the text was changed. This seems to work for player-placed
signs as there is always a rename operation, however AIs can create a sign with text in one
go, in which case the width was never set.

(svn r18781) -Codechange: pass the CommandCost to the callback instead of whether it succeeded or not.
-Fix: AIs did update their last cost incorrectly in network games if the cost of the DC_EXEC phase differed from the ~DC_EXEC phase.