Improve search/find highlighting
Geary currently uses WebKit2's built-in highlighting mechanism, which has two limitations:
- Can only highlight a single string
- Selects as well as highlight the string
These are problems since Geary's search can match multiple terms, and since selected text is used when replying/forwarding to messages.
To work around this, we should update the ConversationWebView and its ConvesationPageState JS class to implement our own search highlighting. This could be using either an existing highlighting lib with a compatible licences like (maybe) https://markjs.io/, or maybe by rolling our own simple DOM+CSS highlighter.