Add default Singer fallback on project import

Currently, when trying to import a project file on a fresh installation of Mikoto Studio will result in an error if the Singer library is empty or no default singer is set. Fixing this should be twofold:

• At startup, check if the default Singer is set. If it isn’t and the library isn’t empty, select the first Singer in the library as default.

• When importing a project, if a default Singer cannot be determined, Mikoto sthould default to the built-in SimpleSynth.