Prepare for changes in wxWidgets 3.3 (#1343)

* 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 https://github.com/wxWidgets/wxWidgets/pull/23449/commits/35c35c235e9c29b40002131602e050dca8d65b8c
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.

src/Main/Application.cpp

@@ -89,7 +89,7 @@ namespace VeraCrypt
 
 	FilePath Application::GetConfigFilePath (const wxString &configFileName, bool createConfigDir)
 	{
-		static wxScopedPtr<const wxString> configDirC;
+		static std::unique_ptr<const wxString> configDirC;
 		static bool configDirExists = false;
 
 		if (!configDirExists)