don't crash when attempting to configure a level with no sides

A similar assertion failure also exists in mp_create I believe.
It's a bit ugly to have to put this in src/game_initialization/
multiplayer.cpp, but probably the best thing if these classes
aren't going to be robust against corner cases like this.
fix bug #22484 (fix random map generation in mp create)

As described in bug report, mp create had a bizarre implementation
for random map vs scenario generation. Scenario generation was called
map generation, and map generation was impossible. We fix it and make
it work like it works in the rest of the game.

At time of writing the wiki describes map generation wml as follows:

```
To use the default [generator] your [scenario] tag must contain one of
the following keys:

scenario_generation=default
map_generation=default

If ‘scenario_generation’ is used, the engine will expect for your entire
[scenario] sub tags to be inside a [scenario] tag inside [generator].
Tags outside of this will be ignored. There may be value in this, but at
this writing, it’s not clear. ‘map_generation=default’ is simpler and
more commonly used. It is also necessary to use this key so that you can
regenerate a map in MP game creation. In its use only generator data is
in the [generator] tag, all other [scenario] data is placed outside of it.
The exception is if you are making an initial MP scenario available in MP
game creation, for this a [scenario] tag must appear inside of
[generator], containing the [scenario] subtags you want to use.
See “data/multiplayer/scenarios/Random_Scenario.cfg” for an example.
```

This commit essentially removes the "exception" pointed out above.
After this, the mp create dialog treats map and scenario generation
both in the "random maps" classification, and it handles them normally,
scenario generation replacing the entire scenario, and map generation
replacing only the map data of the scenario.
put YAMG as a map generator rather than as a scenario generator

this way is a lot less buggy, the other way seems to cause
mysterious crashes
use the "mapgen" log channel rather than engine, for map generators
add lua map generator type
add lua libs for lua map generator, also add bindings for mt rng

Tested to work with this test scenario, and the mainline defaults:

     [label]
         x = {X}
         y = {Y}
         text = {STRING}
     [/label]
[multiplayer]
    id=lua_map_gen
    name= _ "Lua Map Gen Test Scenario"
    description= _ "test test test of lua map gen"
    map_generation="lua"
    [generator]
        id="test"
	config_name="Test Lua Map Generator"
	create_map = << local rng = Rng:create()

			print(rng:draw())
			print(rng:draw())
			print(rng:draw())

			local w = 50
		        local h = 40

			map=""
			for y=1,h do
			  local r = rng:draw() % 2
			  for x=1,w do
                            if x == 10 and y == 10 then
                              map = map .. " 1 "
 			    end
                            if x == (w-10) and y == (h-10) then
                              map = map .. " 2 "
 			    end
			    if ((x + y) % 2) == r then
			      map = map .. "Gg"
			    else
			      map = map .. "Md"
			    end
                            if x ~= w then
                              map = map .. ","
                            end
                          end
                          map = map .. "\n"
                        end
                        return map
                     >>
    [/generator]

    id = foo
    random_start_time=yes

    {DEFAULT_SCHEDULE}
    [event]
        name=prestart
        {LABEL 25 20 ("Lua map generator")}
    [/event]

    [side]
         [ai]
             villages_per_scout=8
         [/ai]
         id=RBY_Side1
         side=1
         save_id=RBY_Side1
         persistent=yes
         color=red
         team_name=Red
         user_team_name= _ "teamname^Red"
         controller=human
         canrecruit=yes
         shroud=no
         fog=no
         gold=1000000
    [/side]
    [side]
         [ai]
             villages_per_scout=8
         [/ai]
         id=RBY_Side2
         side=2
         save_id=RBY_Side2
         persistent=yes
         color=blue
         team_name=Blue
         user_team_name= _ "teamname^Blue"
         controller=human
         canrecruit=yes
         shroud=no
         fog=no
         gold=1000000
    [/side]
[/multiplayer]
add support to create scenarios in lua also
pass the [generator] tag as argument to lua map generator functions
use our loggers in lua map generator, make a better dtor for Rng
create_engine now catches mapgen_exceptions thrown by generators

It puts those messages in the error_message field of the map entry,
so we get nicer gui handling of the problem.
Merge pull request #324 from cbeck88/lua_map_generator

Lua map generator