Moved the definitions of the LCD_STR_* to Conditionals.h

to avoid errors in Marlin_main.cpp. #1860

In the include tree of Marlin_main.cpp the decision between the display types is not made.
To include the right LCD_STR_* ether 'dogm_lcd_implementation.h' or 'ultralcd_implementation_hitachi_HD44780.h' with all their code.
A 'dogm_lcd_implementation.h.h' would be a curiosity.
So i moved both of the definition blocks to conditionals.h

On the long term it could make sense to use the same numbering for the u8glib and the hitachi symbols.