Added a second sensor and made them configurable.

Am I the only one who always need to change the thermistortable.h to add
a dummy table??
Ok, it is done. Just set the thermistor to 999 and it will always read
25C. Great for Testing or Development purposes.

- Fix bug in lcd_control_temperature_menu
- Remove test menu-item left in from multi-line command feature
- Replace literals with translatable strings
- Reduce lcd_preheat code redundancy
- Reduce menu macro redundancy
- Clean up formatting

Last commit before bed.

- Make this work per #1344

- Also apply cleanup for #1514

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

- K8200 uses a non-standard RAMPS with Z_ENABLE=63 ??
- Assuming from previous input that 3DRAG is the same.

The `float` keyword should not be on these vars because they conceal
the outer vars, preventing them from obtaining known values before
being passed in `set_bed_level_equation_3pts`. Possibly related to
`Z_RAISE_BETWEEN_PROBING` not functioning.

When probing PCB´s I had the problem that i needed auto-leveling for specific areas (it´s never the same size).
Not much code, but very useful.
Now I use simply G29 L5 R55 F5 B35 P3 to probe a 4x6cm PCB before milling.
I used int for values... might be wrong...
I hope this time I got the right development tree. (tried first with https://github.com/MarlinFirmware/Marlin/pull/1130/commits)

Fixed how stepper ISR figure it out when the head (extruder) is going to
Min or Max direction.
Added Homing to Max Endstops.

- Naive implementation, assuming the data sheet pin numbers correspond
to Arduino pin numbers.

Fixed delay-times during Init. REPRAP_DISCOUNT_FULL_GRAPHIC_SMART_CONTROLLER won't get initialized on some Boards because delay was to short.

Looks like INVERT_E3_DIR was missing in the configuration.h also as I
did a test compile with 4 extruders and Azteeg X3 Pro defined. So I also
added those lines too. Additional formatting to make the comments line
up better in that section.

Include of language_en.h now behind national language.

All "#define MSG_STR_*" now surrounded by #ifndef and #endif.

+ May decrease the workload for feature developers. They may omit to
update all the language files but "language_en.h". When loading _en
first, all new features give some display, but the new in English.

- May throw some additional warnings about redefinition during compile.