Localise all references to 'mobile'

Remove the mobile property and set it locally in each location
where it is used (preparation for iterating the different
platform+quality combinations which affect the graphics).

Mark each point where a decision is made based on platform+quality
with a plain English version of the combinations which take
effect and when.

Add an explicit hud property which is set from the start screen
and which is independent of the platform.
Merge platform+quality into a single quality value

Added another quality setting (VERY HIGH) and mapped all
combinations of platform + the old quality value to the new
4 value quality option, as follows:

desktop + mid/high quality => 3
mobile + high quality => 2
mobile + mid quality, desktop + low quality => 1
mobile + low quality => 0

All choice points in the code which enabled/disabled
graphics features, based on platform + quality,
were mapped onto a table. This showed which features were
disabled or enabled depending on the platform/quality
combination.

It turned out that mid and high quality resulted in the same
settings on desktop, so they could be captured in a single
value (3).

The only change made to the decision points was that desktop +
low quality previously turned off the booster lighting effect,
whereas that effect is now on for desktop + low quality. This was
done so that mobile + mid quality was equivalent to
desktop + low quality.

By doing this, the "mobile" property can be removed. I also
removed the "half" property, and set it based on quality in the
HexGL class (basically, it's enabled for mobile + low quality,
which is new quality value 0).
Merge pull request #22 from townxelliot/no-mobile-setting-take2-21

Remove platform option and use HUD option instead