Various fixes and optimizations

[bog-dan-kdab]

The patch set contains:

• modernize the code by using c++11 goodies (nullptr, std::function, override, etc.), make use of QString literals
• make the functions faster, especially [un]escapeHtml which now is much faster, drop the Qt5 support.
• make the defaultEscapeMode configurable, by default uses Tag::Escape
• last but not least allow mustache to be used via FetchContent_Declare

[robertknight]

Hello. You've opened a pull request with no description. What are you expecting me to do with this?

[bog-dan-kdab]

Ah, sorry I forgot to add the details :)