Clean and improve this function:
- update rauc installer doc reference
- sanity check input arguments
- fix leaking message with g_autofree (g_variant_lookup() copies the
  string, see [1])
- make error output warning log output
- wrap long lines
- separate nested function calls
- perform empty queue check under lock
- remove unused depth variable, pass NULL instead

[1] https://developer.gnome.org/glib/stable/gvariant-format-strings.html#gvariant-format-strings-strings

Signed-off-by: Bastian Krause <bst@pengutronix.de>

Clean and improve this function:
- update rauc installer doc reference
- sanity check input argument
- simplify if condition

Signed-off-by: Bastian Krause <bst@pengutronix.de>

Clean and improve this function:
- simplify if condition

Signed-off-by: Bastian Krause <bst@pengutronix.de>

Clean and improve this function:
- use g_autoptr for error
- sanity check input argument
- wrap long lines
- turn g_printerr() into g_warning() to make use of logging
- clear RInstaller pointer
- do not call g_signal_handlers_disconnect_by_data() with NULL pointer
  instance if proxy creation failed (goto later notify_complete label
  instead)

Signed-off-by: Bastian Krause <bst@pengutronix.de>

Clean and improve this function:
- remove duplicate doc string
- wrap long lines
- sanity check input arguments

Signed-off-by: Bastian Krause <bst@pengutronix.de>