Complete rmgen object-oriented programming paradigm

[elexis]

The most powerful feature of random map generation is the createArea method which consumes a Placer, Painter and Constraint instances.

The methods to determine a geometric shape, constraining that area arbitrarily and doing something arbitrary to that area are well split by logic into prototypes. This way one can combine them like lego.

Since we now have a significant amount of JS prototypes implementing these interfaces, the files should be split per protoype, gaining the advantages of ​https://en.wikipedia.org/wiki/Information_hiding.

The remaining code would come out nicer too if it would be split into protoypes.

This is a continuation of #4804.

[elexis]

In 21287:

Split the random map generation Placer prototypes into one file per prototype, refs #5042, #4804.
The flags should preserve revision history.

[elexis]

In 21288:

Split random map generation Painter prototypes into one file per prototype, refs #5042, #4804.

[elexis]

In r21289:

Separate object-oriented random map generation core library "rmgen" from miscellaneous helper procedures which are moved to a new library "rmgen-common", refs #4804, #5042.
This way mods can use the placement and painting prototypes while excluding any common 0ad code.

[elexis]

In 21290:

Use CamelCase for random map files containing the prototype of the same name, refs #5042, #4804.

[elexis]

I'm a bit reluctant to split the 11 constraint prototypes having 160 lines into 11 files with 14 lines each. Same for Terrain and Group. player.js and the wall builder should likely become a protoype eventually, but aren't part of rmgen anymore.