Use the same sizing approach we use for GtkEntry:
ignore icons when measuring. This ensures that
password entries don't change size as icons come
and go.
It can be a bit confusing to have an indicator
and an action next to each other, and with the
peek icon, the need for the Caps Lock warning is
reduced, since you can just reveal the text to
see that it is capitalized.
Therefore, only show the Caps Lock warning if
the peek icon is disabled.
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.
We don't want it to appear clickable, but we still
need to keep it pickable for the tooltip to work,
so explicitly give it the same cursor that we use
for the text.
