Fix navigation bar title and subtitle appearance

[rekola]

I added a method setTitles, which sets both titles at the same time, to help with the following changes.

• Title font doesn't look similar to the default navigation bar font. At least, it should be bold.
• Subtitle font should look similar to the subtitle font that Whatsapp has. At least, it should be light, small and grey.
• If the subtitle is empty, only title should be shown, and it should look similar to the normal title. At least, it should be vertically centered.
• The new title doesn't show as much text as the default UINavigationBar title. There is much space around the title, but still the title is cut, and has ellipsis.
• Title positioning uses the size of self.frame, so it probably doesn't respond nicely to orientation changes

[rekola]

Fixed.