Introduced lcd_strlen() and lcd_strlen_P().
Replaced the old functions where necessary.
Reworked language_ru.h.

Speeded up test for zero length string in cardreader.cpp

This won’t reduce the string storage in the end, but it makes the
language files smaller. Also removed remaining unused strings.

* Depending on extruder count, will add menu items for ultralcd to edit
individual PID parameters for each extruder
* Added menu items to each language_xx.h
* Builds OK, but recommend testing with typical LCD

This reverts commit 2b9722f387.

No matter what LANGUAGE_CHOICE was set to it would always take the en branch.
I revamped it a bit to use a macro to create the language include file name. This required a slight tweak to the RU declaration since it's special-cased in a couple places in code.

This is an attempt to pull my changes from #1222 over.

To test/enable, uncomment the #define DELTA_CALIBRATION_MENU in Configuration.h

Breaking up languages.h makes it easier to translate. Using language
codes makes it easier to select a language and sets the naming standard.