Disallow more unsafe string->path conversions allowed by path append operators

Add more fs::path operator/ and operator+ overloads to prevent unsafe
string->path conversions on Windows that would cause strings to be
decoded according to the current Windows locale & code page instead of
the correct string encoding.

Update application code to deal with loss of implicit string->path
conversions by calling fs::u8path or fs::PathFromString explicitly, or
by just changing variable types from std::string to fs::path to avoid
conversions altoghther, or make them happen earlier.

In all cases, there's no change in behavior either (1) because strings
only contained ASCII characters and would be decoded the same regardless
of what encoding was used, or (2) because of the 1:1 mapping between
paths and strings using the PathToString and PathFromString functions.

Co-authored-by: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>

src/qt/guiutil.cpp

@@ -508,7 +508,7 @@ fs::path static StartupShortcutPath()
         return GetSpecialFolderPath(CSIDL_STARTUP) / "Bitcoin.lnk";
     if (chain == CBaseChainParams::TESTNET) // Remove this special case when CBaseChainParams::TESTNET = "testnet4"
         return GetSpecialFolderPath(CSIDL_STARTUP) / "Bitcoin (testnet).lnk";
-    return GetSpecialFolderPath(CSIDL_STARTUP) / strprintf("Bitcoin (%s).lnk", chain);
+    return GetSpecialFolderPath(CSIDL_STARTUP) / fs::u8path(strprintf("Bitcoin (%s).lnk", chain));
 }
 
 bool GetStartOnSystemStartup()
@@ -589,7 +589,7 @@ fs::path static GetAutostartFilePath()
     std::string chain = gArgs.GetChainName();
     if (chain == CBaseChainParams::MAIN)
         return GetAutostartDir() / "bitcoin.desktop";
-    return GetAutostartDir() / strprintf("bitcoin-%s.desktop", chain);
+    return GetAutostartDir() / fs::u8path(strprintf("bitcoin-%s.desktop", chain));
 }
 
 bool GetStartOnSystemStartup()