MidiSettings: Don't leak settings in failure case.

CID 1422944.

Use driver settings API for the MIDI settings

Suggested by Adrien, to make the MIDI settings more future proof when
more settings will be added, and to make manual editing less error prone.
Moved th

Use driver settings API for the MIDI settings

Suggested by Adrien, to make the MIDI settings more future proof when
more settings will be added, and to make manual editing less error prone.
Moved the settings from B_USER_SETTINGS_DIRECTORY/midi to
B_USER_SETTINGS_DIRECTORY/Media/midi_settings.

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Fix loading of MIDI SoundFonts with spaces in filename

Midi: Remove some duplicated code
Introduced new private read/write_midi_settings() and used them
in MidiSettingsView and SoftSynth.

Merge branch 'master' into app_server

Conflicts:
src/kits/app/Roster.cpp