*: improve multiple things e.g. making some strings.cc functions modify strings in-place, improving m4_ax_have_qt.m4 code, making anime_db.cc rely on std::optional rather than std::shared_ptr (which was stupid anyway)

# Icons
These icons are from Yusuke Kamiyamane's [[Fugue Icons]](https://p.yusukekamiyamane.com/) pack. The original files have been left intact alongside bin2h variants (for easy inclusion in source code).