Speech synthesis, Night mode, Remember reading position, ...

* speech rate can be modified on per-language basis, but only in source codes; there's no interface for that

Targets these issues:
* #23 
* #12 
* #25 
@gyim 
> I noticed an odd behavior about this option: if it is disabled, text-to-speech works fine, but when it is enabled, it stops working.

The `webViewDidFinishLoad` wasn't called when the content was the same. I added a forced call in such cases.

I've found an AVSpeechSynthesizer bug for which I added a work around in the following commit. The synthesizer did work only on first attempt, in next prayers it just didn't work. 

> Also, how about hiding this option? If this parameter is automatically enabled by pressing "Read", I see no reason why a user would enable this manually.

I discussed this with @breviar-sk and he told me to do it this way. I suggested hiding the option, but Juraj had some more-or-less valid arguments for leaving it there :) Blind-friendly=ON means hiding rubrics and various other things and he thought it might be useful for non-blind / non-TTS users as well. Hard to tell now.