app/SettingsBase: Fix systemsettings unable to start when missing plugin
Switch to alternate view if the current view is missing on startup.
BUG: 451054 FIXED-IN: 5.24.3
Test: Switch to sidebar mode, close systemsettings and run mv /usr/lib64/qt5/plugins/systemsettingsview/systemsettings_sidebar_mode.so{,.bak}
, and run systemsettings.
Edited by Fushan Wen