Encapsulate Widget.get_children() → ready for GTK4

GTK4 drops gtk_container_get_children(), and gtkmm4 dropped the method
too, without considering that it perhaps can/should add its own helper.

Iʼve asked for the method⁂ , but until (if) that is accepted, we can and
should just add our own get_children(Widget&) that can encapsulate this
and avoid us having to change every former call site during The Big Pile
of Changes for GTK4: instead, we can change/verify those now & GTK4 will
only need us to change the implementation at one place in our new helper

While here, I do various sorely needed clean-ups:
* parameter: Return ptr-to-const member tip, not modifiable one. + FIXME
• parameter/togglebutton: fix refresh_button() did not actually update
  the image, instead just creating and discarding a different GtkImage!!
• LPEFilletChamfer: move things instead of copy & exit early not indent
• LPETransform2Pts: deduplicate horrid code, & shadowing of local `tip`
• various LPEs: ditto above, check we got managed widged, yadda yadda
• document-check: Fix leaked GLib string. Deduplicate code to run dialog
• DialogBase: Fix copy-pasted type cast in fix_inner_scroll(), & tidy up
• etc.

⁂  https://gitlab.gnome.org/GNOME/gtkmm/-/issues/143