GitLab

* change needed to satify Rust 2021
* lint update
Signed-off-by: Ralf Zerres <ralf.zerres@networkx.de>

* substitute assert_eq!() with assert!()
  to be in line with Rust 2021
Signed-off-by: Ralf Zerres <ralf.zerres@networkx.de>

* clippy suggested to remove unneeded borrow
Signed-off-by: Ralf Zerres <ralf.zerres@networkx.de>

New example code `mgs_handler`

This example illustrates the usage of massage handling between
individual widgets (here: SenderView and ReceiverView).

* msg_handler: the controlling window.
* sender: handle view and state to handle message sending.
* receiver: handle view and state to react when message are passed over.
Signed-off-by: Ralf Zerres <ralf.zerres@networkx.de>

Gtd

The project no longer uses OrbTk.

Use reference-counted stylesheet when rendering

+ `Context` takes ownership of the theme in `Context::switch_theme()`
+ `RenderObject::render()` makes use of the reference-counted theme stored
  in the Entity Component System
+ `RenderObject::render_children()` likewise passes immutable references to
  the theme
+ Updated `showcase` example to match

Shell and ComboBox updates

* overcome lint error
Signed-off-by: Ralf Zerres <ralf.zerres@networkx.de>

* remove duplicate definition assignment for
  `build_context.register_shared_property::<f32>
Signed-off-by: Ralf Zerres <ralf.zerres@networkx.de>

* rearrange enum 'Key' and impl 'From<Key>' in lexical order
* add documentation for public definitions
* rearrange key_event.scancode in lexical order
* add Tabulator identifier key (K_TAB)
Signed-off-by: Ralf Zerres <ralf.zerres@networkx.de>

documentation update

api: description clarification when switching entitycontext with `change_into`
widgets: typo correction in `pager`
Signed-off-by: Ralf Zerres <ralf.zerres@networkx.de>