Fix manifest bool setting parsing

[nullket]

• kodi boolean settings will be given as strings with 'true' or 'false'
• I assumed them to be of type bool instead. The manifest settings is therefore always true
• Once we add more boolean settings (or do refactoring) we should think about a functional wrapper e.g. other plugins have something like this

Solves the first half of https://github.com/firsttris/plugin.video.sendtokodi/issues/77

I am merging it directly into master because the current state is broken and this fix is small.