Add FlexibleToolbarLayout

[taltstidl]

The Design Support Library has a component called CollapsingToolbarLayout which is quite cool when used with a parallax image and only text. I can think of a few annoyances though, that I'd like to fix in my own version of this (FlexibleToolbarLayout):

• The CollapsingToolbarLayout only allows text to be scaled. While this is enough for some cases, I'd like to be able to use an image, a title and a subtitle (much like in Telegram Messenger).
• The CollapsingToolbarLayout doesn't render a shadow when not completely collapsed, even when there's no image involved, just a solid color.
• The CollapsingToolbarLayout doesn't respond to touches within itself.

[taltstidl]

This is implemented now. The only thing left are a few minor improvements (better rtl support, methods for manually expanding and collapsing maybe) which have been migrated to Issue #9.