TabWidgets couldn't be used in GML properly, as the GML creation
routines didn't actually call the necessary functions in the TabWidget
to get a new tab added. This commit fixes that by making the name of the
tab a normal property, the previously introduced "title", which can be
trivially set from GML. Therefore, try_add_widget() loses an argument
(while try_add_tab doesn't, because it newly constructs the widget).
This allows us to get rid of the silly "fixing my widget tree after the
fact" code in Help and will make it super easy to use TabWidget in
future GML. :^)
Converts Help's layout to GML, propagates model and icon creation
errors, and switches to the MainWidget-namespace organization
pattern seen in more up-to-date apps like TextEditor to make things
easier to maintain going forward.
