* Move from deprecated wxScopedPtr to std::unique_ptr
wxScopedPtr was included previously through some header hierarchy which as of 3.3 is
no longer the case causing it to break. But instead of including a header for a deprecated
function explicitly, just move to std::unique_ptr as recommended by upstream.
* Convert to explicit conversions from wxString
As of 35c35c235e
wxWidgets defaults to STL classes, which has a side-effect that
some implicit conversions break. This patch converts those conversions
to explicit in anticipation of wxWidgets 3.3 release.
Adhere to XDG Desktop Specification and use the environment variable
XDG_CONFIG_HOME to determine location of configuration files on *all*
platforms. If it is unset or empty resort to platform-specific defaults.
On Windows and OS X, `wxStandardPaths` provides correct defaults (equal
to the previous hard-coded paths) but on Linux and other Unices
`~/.config/appinfo` would be better than `~/.appinfo`. This means we
treat those platforms as a special case. It also means that we may need
to fall back to the legacy location if it exists but the new location
doesn't.
