Formspec cleanup and improvements

[OgelGames]

This is mostly a code cleanup PR, but with a few formspec changes and improvements.

• Moved fs_helpers to a separate file, cleaned-up existing code, and added several new helper functions
• Cleaned-up formspec code using the new helper functions.
• Standardized all formspecs and reduced the amount of text buttons.
• Improved i3 and mcl_formspec compatibility with correct styling.
• Replaced solid color images in sorting tube formspecs with boxes.

The stack-wise injector is the best example of the visual changes, since it includes one of every element. Top row is before, bottom row is after:

Fixes #205, closes #208

[wsor4035]

we really should make these dynamic instead of hardcoded meta

[The4codeblocks]

we really should make these dynamic instead of hardcoded meta

explain?

[wsor4035]

we really should make these dynamic instead of hardcoded meta

explain?

core.show_formspec instead of encoding it in meta

[The4codeblocks]

we really should make these dynamic instead of hardcoded meta

explain?

core.show_formspec instead of encoding it in meta

well that's with the compromise of not having the forms autoupdate

[OgelGames]

I still don't like the idea of any node formspec being dynamic unless it needs to be.

Good examples are fancyvend and travelnet, because in both cases the formspecs need to be different depending on who looks at it.

For pipeworks nodes, which only need to update if you change a setting, should not be dynamic in my opinion.