Do not ignore non-visible widget when setting a map.

At the time SetTextFromMap is called; the widget may not yet be visible but may be later (like if its state depend on another element not yet available).
All any text widgets to use templates, not just the one whose name are defined in the map

This allows to use more creatively the widget dependencies, without being bound to specified object names
Allow widget dependencies to work with progress bar.

Only images and textareas could be used previously.
Ideally, dependencies should work with any ui objects based on their visibility status. But trying to keep changes to a minimum.
Leave visibility status of progress text in notification up to the theme.
Update notifications default theme.

Using new dependencies and template behaviour. Make description text be multi-lined if there's enough room
change const-ness of class members as appropriate
change const-ness of arguments as appropriate
Fix notifications not using their allocated screen should the client unregistered before notifications queued could be processed.
Fix notification leaks should screen failing to be created...
Fix crash when exiting the fronted under some circumstances

Should the frontend quit while a client had registered but never displayed a notification would have resulted in a null dereference.
Fix whitespaces
Do not display "backend is online" notification, if no errors were displayed before
Restore previous backend disconnection message now that the notification theme can handle longer description text.

There's an extra space in it that could have been removed. Little bit less work to translate new strings.