Use only one QSettings file, and name it correctly on Mac OS X

**75923a7 correct QSettings file name on Mac OS X**
On Mac OS X the (main) preferences file had a strange name instead of the expected `org.frescobaldi.frescobaldi.plist`. After this commit, the file is named following the usual conventions (domain name in reverse order and application name).

**7c2a9bb use only one QSettings file**
The most part of Frescobaldi's settings were written to a single QSettings file with different groups but some settings were written to different files. The way these different files (other than the main file) were managed was semantically incorrect: according to Qt documentation, QSettings files should be named after the organization and the application name. After this commit, a single QSettings file is used, with the previous different files becoming different groups in that unique file, and all QSettings object initializations are argument-less.

**Settings upgrade**
The function `transferSettings` takes care of upgrading the QSettings locations in both commits, so there shouldn't be any loss of previous settings. When it is called, `transferSettings` checks if the specified settings need to be transferred, so the actual transfer is performed only once.
If it is preferable, the function definition and calls can be easily separated into a dedicated script that the user can launch if needed.