This fixes a weird gap showing between the "Visible as [...]" and
the devices list.

When there are no devices, the list doesn't display
the "Searching for devices..." placeholder. Instead,
a broken GtkListBox is shown.

Fix that by using a GtkListBox placeholder to show the
"Searching for devices..." message.

Closes https://gitlab.gnome.org/GNOME/gnome-bluetooth/-/issues/113

When removing a device, the text shown in the confirmation
dialog has some problems:

- "Remove “%s” from the list of devices?" is too long and
also makes the confirmation dialog to have a very long
width

- The dialog uses an outdated writing style: "remove" is
being used instead of "forget". By the way, "forget" is
already being used by the button that triggers this
dialog (the "Forget Device..." button)

To fix these issues, update the dialog heading and body
strings according to the mockup from [1].

[1] https://gitlab.gnome.org/GNOME/gnome-bluetooth/-/merge_requests/185#note_2089868

Closes https://gitlab.gnome.org/GNOME/gnome-bluetooth/-/issues/149

(cherry picked from commit 6347c0c7017fee6846e351c1793ee05dcbf39387)

(cherry picked from commit bb5cdca075c49a32d5ae3cf93818426a24b5b837)

AdwStatusPage isn't mean to act like a whole container for the page; this
can break scrolling as AdwStatusPage contains a GtkScrolledWindow. Also,
there is already AdwPreferencesPage, which is the proper widget to use in
this case.

Use AdwPreferencesPage and AdwPreferencesGroup instead.

Also, port test-settings to AdwWindow, otherwise it
won't open the device properties dialog.

Just for cosmetics.

Use icon name fallback to set a very precise icon name, that will
automatically fall back to a less precise one if not available.

This fixes the phone icons in the Bluetooth Settings appearing as
generic symbolic icons rather than colourful ones.

Port the following buttons to AdwButtonRow:

- "Mouse & Touchpad Settings"
- "Sound Settings"
- "Keyboard Settings"
- "Send Files..."
- "Forget Device..."

lib/meson.build:180:4: ERROR: requires argument not a string, library with pkgconfig-generated file or pkgconfig-dependency object, got <InternalDependency dep140487840149728: True>