qt: Do proper boost::path conversion

Convert from QString unicode from/to the OS-dependent locale
as used by boost::filesystem::path as needed.

Solves #3916.
qt: Only override -datadir if different from the default

Fixes #3905.
Merge pull request #3935

c61fe44 qt: Only override -datadir if different from the default (Wladimir J. van der Laan)
7e591c1 qt: Do proper boost::path conversion (Wladimir J. van der Laan)