Battery handling, machine loop optimization

[S-S-X]

Requires some testing and opinions.

• Battery box formspec updated, power meter wont update while formspec is open.
• Default digiline channel is empty and channel field moved to main form, second formspec removed.
• Stack splitting removed from battery box, stacking items that can be charged without taking whole stack capacity into account is cheating anyway. Also I've not yet seen stackable rechargeable items...
• Battery box node swapping / texture update done with nodetimer instead of globalstep, reduces a lot of node swapping (every second in active network vs every 2 seconds in active area).
• Optimized technic machine run loop.
• Getting smaller pull requests merged more often is better for quality and development speed.

So next thing would be to find problems in this pr and then continue with things not yet done, this is not enough to close any of issues alone.

[BuckarooBanzay]

Merged the PR locally and played around with a "simple" solar-powered battery-buffered grind-and-smelt loop
which i multiplied by x100:

A few performance stats:

• with this PR: 20ms / second
• without this PR: 65ms / second

This may have huge potential if applied in scale (like with lots of players 😉)

[S-S-X]

Tested a bit, tool charging / discharging, digiline, infotext, formspec power meter (with limitations mentioned in #112) and texture/node updates works.
Could probably merge this too unless anyone sees any big problems here, tests done with changes to current master atow (like if it was already merged).

[S-S-X]

One slightly annoying thing this PR introduced was slight delay for battery box front texture update (power meter) after leaving area for some time and then getting back there later.
This is more visible if batteries drained to empty from full/almost full or charged to full from empty/almost empty, can easily see outdated power meter texture that immediately jumps to empty or full when nodetimer executes next time.

Not really big issue but very noticeable especially with some great wall of batteries.