Support for multiple files

[mcendu]

This library is completely unable to tolerate being included in multiple files. Attempting to include the library header `LCDI2C_Multilingual in multiple files of a sketch results causes link failures as follows:

ld.exe: <main>.cpp.o:<libraries>\LiquidCrystal_I2C_Multilingual\src/ROM/
  Generic.h:28: multiple definition of `GenericROMLetters';
  <aux>.cpp.o:<libraries>\LiquidCrystal_I2C_Multilingual\src/ROM/Generic.h:28:
  first defined here
ld.exe: <main>.cpp.o:<libraries>\LiquidCrystal_I2C_Multilingual\src/Custom/
  Vietnamese.h:24: multiple definition of `VietnameseCustomLetters';
  <aux>.cpp.o:<libraries>\LiquidCrystal_I2C_Multilingual\src/Custom/
  Vietnamese.h:24: first defined here
ld.exe: <main>.cpp.o:<libraries>\LiquidCrystal_I2C_Multilingual\src/Custom/
  Symbols.h:21: multiple definition of `SymbolCustomLetters';
  <aux>.cpp.o:<libraries>\LiquidCrystal_I2C_Multilingual\src/Custom/
  Symbols.h:21: first defined here
ld.exe: <main>.cpp.o:<libraries>\LiquidCrystal_I2C_Multilingual\src/ROM/
  Katakana.h:74: multiple definition of `KanaROMLetters';
  <aux>.cpp.o:<libraries>\LiquidCrystal_I2C_Multilingual\src/ROM/
  Katakana.h:74: first defined here
ld.exe: <main>.cpp.o:<libraries>\LiquidCrystal_I2C_Multilingual\src/ROM/
  Katakana.h:38: multiple definition of `KanaDiacriticCharacters';
  <aux>.cpp.o:<libraries>\LiquidCrystal_I2C_Multilingual\src/ROM/
  Katakana.h:38: first defined here
ld.exe: <main>.cpp.o:<libraries>\LiquidCrystal_I2C_Multilingual\src/ROM/
  RussianLatin.h:23: multiple definition of `RussianLatinROMLetters';
  <aux>.cpp.o:<libraries>\LiquidCrystal_I2C_Multilingual\src/ROM/
  RussianLatin.h:23: first defined here
ld.exe: <main>.cpp.o:<libraries>\LiquidCrystal_I2C_Multilingual\src/ROM/
  Russian.h:24: multiple definition of `RussianROMLetters';
  <aux>.cpp.o:<libraries>\LiquidCrystal_I2C_Multilingual\src/ROM/
  Russian.h:24: first defined here
ld.exe: <main>.cpp.o:<libraries>\LiquidCrystal_I2C_Multilingual\src/ROM/
  Latin.h:25: multiple definition of `LatinROMLetters';
  <aux>.cpp.o:<libraries>\LiquidCrystal_I2C_Multilingual\src/ROM/
  Latin.h:25: first defined here

The offending files hold definitions for conversion tables, while common practice is to place the definitions in a .c/.cpp file and mark the header declarations extern.