[core] Fix Warnings and Regenerate Splash Screens (#186)

This commit fixes the following warnings for the web build:

```
Warning: In index.html:38: Local variable for "serviceWorkerVersion" is deprecated. Use "{{flutter_service_worker_version}}" template token instead.
Warning: In index.html:60: "FlutterLoader.loadEntrypoint" is deprecated. Use "FlutterLoader.load" instead.
```

To re-add the slash screen we alsow run the
`flutter pub run flutter_native_splash:create` command again, which also
updated the splash screens for iOS and Android.

Last but not least this commit also fixes the following warnings for the
Windows build:

```
windows/runner/flutter_window.cpp does not ensure the show window callback is called, updating.
windows/flutter/CMakeLists.txt does not use FLUTTER_TARGET_PLATFORM, updating.
```

app/windows/runner/utils.cpp

@@ -45,9 +45,11 @@ std::string Utf8FromUtf16(const wchar_t* utf16_string) {
   if (utf16_string == nullptr) {
     return std::string();
   }
-  int target_length = ::WideCharToMultiByte(
+  unsigned int target_length = ::WideCharToMultiByte(
       CP_UTF8, WC_ERR_INVALID_CHARS, utf16_string,
-      -1, nullptr, 0, nullptr, nullptr);
+      -1, nullptr, 0, nullptr, nullptr)
+    -1; // remove the trailing null character
+  int input_length = (int)wcslen(utf16_string);
   std::string utf8_string;
   if (target_length == 0 || target_length > utf8_string.max_size()) {
     return utf8_string;
@@ -55,8 +57,7 @@ std::string Utf8FromUtf16(const wchar_t* utf16_string) {
   utf8_string.resize(target_length);
   int converted_length = ::WideCharToMultiByte(
       CP_UTF8, WC_ERR_INVALID_CHARS, utf16_string,
-      -1, utf8_string.data(),
-      target_length, nullptr, nullptr);
+      input_length, utf8_string.data(), target_length, nullptr, nullptr);
   if (converted_length == 0) {
     return std::string();
   }