[WIP] Add a locale/language setting/option to Sonic Pi and organise some GUI code

[SunderB]

I've tried to start work on making a language setting, so the user can change the locale of Sonic Pi individually from the system locale.

Also, while adding the option, I've organised some parts of the code to hopefully make it easier to read, and to add to in the future. (So this also acts as an updated version of PR #1875) I just hope that I haven't accidentally missed/removed anything! If you don't like how I've organised parts of the code, I can revert the organisation changes if needed.

[SunderB]

I hope this is a good start on the language option :)

[SunderB]

In order to build it on my Debian 9 laptop, I had to change SonicPi.pro slightly so it could find qscintilla2:

# Linux only
unix:!macx {
  LIBS += -lrt -lqt5scintilla2
  QMAKE_CXXFLAGS += -std=gnu++11
  QMAKE_CXXFLAGS += -Wall -Werror -Wextra -Wno-unused-variable -Wno-unused-parameter
  debug {
    QMAKE_CXXFLAGS += -ggdb
  }
}

I changed the locale setting manually in the sonic-pi settings file, and it seems like Sonic Pi loads the correct locale from the settings when starting, which is good! The locale prefs combo box also lists the different available languages (by their English names at the moment; in the final version it'll show their native names), and an option to use the system locale. :)

But I was unable to test if it saved the locale setting correctly, as Sonic Pi keeps crashing with a segmentation fault when I try to close it; I don't know if that's to do with the code I've added, other code in SPi, or something else.

[samaaron]

Apologies, we've now moved our default branch from master to main. Please consider creating a new pull request merging with main with these changes.

I am closing this not out of any lack of interest, but in order to ensure new code moves to our new default branch. Thanks!