0.9.0

This releases is quite large and includes some breaking changes:

• Cursive::new() has been renamed Cursive::default(). Cursive::new is now used to select a backend, allowing third-party backends.
• Scrolling is now handled by a new view wrapper, ScrollView. For TextView, SelectView and ListView, it is opt-in, by wrapping it in a ScrollView or calling .scrollable() from the Scrollable trait.
• Cursive::cb_sink() now returns a crossbeam_channel::Sender instead of chan::Sender. The API is mostly similar.
• SelectView::selection now returns a Option<Rc<T>> instead of panicking on an empty list.
• The features of AnyView have been merged into the regular View, so you should not need to use AnyView directly, and can only use View.
• Vec4 has been renamed to Margins.

In addition, many non-breaking changes are present as well:

• Callbacks sent through Cursive::cb_sink() are now processed instantly, without the need for set_fps.
• Backends are now a dynamic choice rather than a comile-time selection.
  • This means library do not need to bother with backend features anymore
  • Features are still there to add available backends, but they are mutually compatible.
• Events are batch-processed, resulting in higher performance when a lot of events are sent at once (for instance when scrolling with the mouse).
• Some methods now return a callback:
  • SelectView::{set_selection, select_up, select_down, remove_item}
  • EditView::{set_content, insert, remove}
• API additions:
  • StackView::find_layer_from_id
  • SelectView::insert_item
  • TextArea::{enable, disable}
  • rect::Rect
• Various bugfixes