API updates

• SelectView::{item, with_all} now accept S: Into<StyledString> for colored labels.
• Add ScrollView::scroll_to_important_area.
• Add LinearLayout::set_focus_index.
• Add XY::{sum, product}.
• view::scroll is now a public module.
• Add Cursive::process_events and Cursive::post_events.
  • This gives users finer control than Cursive::step.
• Layer now has a color option.
• LinearLayout can now directly add boxed views without re-boxing.
• Add inner getters to EnableableView.
• Add PaddedView::get_inner(_mut).
• Add a bunch of constructors for Margins.
• Add Dialog::padding_lrtb
• Add Dialog::set_padding*
• Add PaddedView::lrtb

Improvements

• Changed the default color for TitleSecondary from yellow to light blue.
• Changed the default color for Tertiary from grey to white.
• Reduced dependencies (toml is now optional, removed hashbrown).
• Cursive::default() now fallbacks do dummy backend if no other is available.

Bugfixes

• Fixed ScrollView::show_scrollbars().
• Correctly update the offset for ScrollView after focus change.
• Fixed layout for BoxView with some size constraints.
• On Windows, do not print unix-specific character during initialization.
• Fix out-of-bounds access for some mouse events in MenuPopup