Revert "Adwaita: axes borders of notebooks inside frame"

This reverts commit 01e6811776cf6ad8738f182972edbb84626d7fae.
This assumption doesn't work latelly
widget-factory: set show-border to false on page 3 notebooks
Add private API to query tick callbacks

This information can be useful for debugging purposes.
Inspector: Show more misc info about widgets

This adds clip area, accessible role, mapped, realize,
is-toplevel, child-visible, mnemonic labels and tick
callbacks to the displayed information.
widget-factory: Add mnemonics in the prefs dialog

This helps testing the mmemonic label display in the inspector,
and is just good practice.