Fix decimal separator handling when loading real numbers

[PhysSong]

• Remove broken DataFile::LocaleHelper.
  When saving float values into QDomElement, Qt uses QString::setNum. As of Qt 4, the function always uses the C locale, so we don't need to worry about decimal separators when saving files.
  The problem is Qt5 doesn't provide a way to handle both period-separated(1.2) and comma-separated(1,2) real numbers in one function call because QString::toFloat always use the C locale only. QLocale::toFloat always tries its specific locale only.
• Add new LocaleHelper.h and use it
  To solve the problem, I created a function which tries both period and comma. Using it will work on any platforms.

Fixes #4442.

[JohannesLorenz]

I could code-read and test this branch. Let me know when it's ready.

[PhysSong]

I used inline keyword to avoid errors for now. Should I move the definition to a source file instead?

[JohannesLorenz]

@PhysSong Sounds good. Also maybe add doxygen-like comments so one can guess what the functions do by just looking at the header.

[PhysSong]

There are some functions defined in other header files. I think my helper functions are not too large and it's fine to leave them inline.

Also maybe add doxygen-like comments so one can guess what the functions do by just looking at the header.

Updated the description at the beginning.