Re-factorization of mappers to have a homogeneous format

[SergioMartin86]

In this PR, I took all the mappers dispersed along multiple cpp files and put them into a single, homogenous format that allows you to quickly see what mappers are supported by QuickNES. In a nutshell, the benefits are:

• All mappers are enabled by default (as opposed to having to run 'register_extra_mappers', 'register_misc_mappers' separately.
• Removed the contrived 'register_mapper' function that consisted of a map of code->functions that created new mappers. Now the 'Nes_mapper.cpp' file is much simple and cleaner
• Added Mapper 70, as one possible variant of 152 -- required for certain versions of games like Arkanoid II
• Some warnings were cleaned here and there (wrong sign comparison, etc).
• With the new 'mappers' folder containing all the mapper header files, the file structure of QuickNES looks much cleaner, where only NES emulation-related files exist in the root folder.

[keithbowes]

It's nice that someone has taken an interest in this. Sure, it has its bugs (minor visual glitches, audio/video sync problems when recording) and limitations (no peripheral support), but just for playing gamepad-oriented games, it's perfectly adequate.

[hizzlekizzle]

It's also very valuable for extremely low-power devices, like PS2. So, yeah, echoing @keithbowes in voicing appreciation for the attention :)

[SergioMartin86]

I'm glad to see there's interest in the development of quickNES. I found it to be a fantastic emulator, and it's pretty accurate too!

I've created a new repository (https://github.com/SergioMartin86/quickerNES) to start serious work on it and see if it can be made even better.