These were showing up as missing icons when opening
the Inspector with the hicolor icontheme:
system-search-symbolic
go-previous-symbolic
go-next-symbolic
display-brightness-symbolic
Add a ::show-peek-icon property and show a clickable
icon when it is set. Clicking it toggles the visibility
of the content. The same functionality is also accessible
via a context menu item.
This is a common feature of password entries.
This subdirectory gets in the way when integrating the inspector
build more fully with GTK+, and does not really add anything.
Just move everything one level up.