GUI was storing selected language as index to the languages list.
This is fragile since the order and count of items can change.
This commit changes to using ISO language code (e.g. "en" for
"English").
Fixes ticket #2446 (GUI: Don't use index number for language selection)
Instead of separate language name and filename lists use one
list of structs. This makes language name and filename more
connected and makes handling of them easier.
Also added QSetting value names to common.h
Work is halfway on both translations and .ui files.
I added a very quick and rough finnish translation.
The program now requires the translation files to be created before running
that can be done with lrelease gui.pro.
To compile the whole GUI one must do the following
cd gui
qmake
lrelease gui.pro
make
