Implement Cellular panel for modem management

Epic: https://source.puri.sm/Librem5/use-cases/issues/2

A cellular/wwan panel is required for managing modems. The following items are to be done:

• Show different settings page per connected modem
• Handle Airplane Mode (inform Users if Airplane Mode, and allow users to disable it there)
• Show sim missing page if sim is missing
• Add a way to unlock SIM locked (before the actual settings are shown)
• Allow users to change modem mode (2G/3G/4G etc)
• Allow users to register to network
  • Automatically
  • Manually
• Add a way to enable/disable data per modem
• Add a way to enable/disable modem on boot
• Add a way to select the default modem to use for data

Since ModemManager itself doesn't have any backend to store the settings, we will have to figure out where the settings should be saved. The most appropriate candidate seems to be g-s-d. Data related settings can be saved in NetworkManager. But it have to be decided where the per device/sim settings should be saved.

Dependencies

• Implement Access Point settings
• SIM Lock
• Date and Time Timezone setting screen cannot be used or canceled
• G-c-c doesn't start via ssh/serialmonitor
• Cannot change network mode in the Cellular panel
• Disabling suspend after 1 minutes of idle remove the option to re-enable for 1minute
• Settings (and Clocks) Keyboard Shortcuts overflow (is it needed on mobile?)
• Settings Users Automatic Login disabled (should it even be there?)
• Settings Date & Time Time Zone Map Select with Keyboard back button and scrolling issues
• Settings Details About has invalid and incorrect values
• Settings Power Button Action not valid
• Swipe toggle buttons not working, but tapping does
• Settings Region & Language Input Sources gear icon shows "Keyboard Shortcuts" probably meant for desktop/laptop not phone
• Settings Region & Language 'Eye' icon does nothing
• Changing Language offers "Restart..." button that doesn't work
• Settings Region & Language Broken Icon
• Settings Search broken image
• Design Notifications Icons Fuzzy for Color, Date & Time, Network, and Power