#161: Use QString::fromStdString(x) instead of QString{x.c_str()}

[aaronhktan]

Description

• Using QString::fromStdString() should be a little bit more efficient when converting from an std::string to a QString. (See here.)
• Also replaces an instance of const std::string & with std::string_view.

Part of improvements as part of #161.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Tested on macOS.

Checklist:

• My code follows the style guidelines of this project (black for Python
  code, .clang-format in the src/jyut-dict directory for C++)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have translated my user-facing strings to all currently-supported languages
• I have made corresponding changes to the documentation