Add kas_widgets::edit::Editor

[dhardy]

Move much of the internals of EditField to a new struct, Editor. This allows EditGuard methods to take receiver &mut self and &mut EditGuard instead of &mut EditField. This may also be useful for code-share with other editor widgets.

EditGuard methods are tweaked:

• focus_gained is called when the first of keyboard or IME focus is obtained
• focus_lost is called when both are lost
• update is called only without input focus
• focus_lost calls update in the default impl