Implement generic browser widget — radicle-tui

add support for ViewState::Any(Box<dyn Any>)
implement Widget::view_state() and return the view’s state
implement Container::view_state() and return the content’s widget state
browser widget Browser uses BrowserState<Item, Filter> internally
Browser handles events and updates the internal state
Browser::view_state() returns ViewState::Any(BrowserState as Any..)
BrowserProps holds an optional BrowserState to set the internal one, supports BrowserProps::state() builder
Remove all app-specific message types that are internal to the browser e.g. CloseSearch or SelectIssue
app messages type Message only knows BrowserChanged { state: BrowserState }