Ubo-app

- hotfix: remove a dangling whitespace in `install.sh`

- feat: add `ubo-pulseaudio.service` to take care of keeping pulseaudio up
- refactor: make sound service work with or without pulseaudio installed
- refactor: organize system related codes and assets

- fix: enable i2c and spi using `raspi-config` in `install.sh`
- fix: reboot after installation (needed for audio driver)
- fix: add user to netdev group in `install.sh`
- fix: install `python3-picamera2` and `python3-dev` debian packages in `install.sh`
- fix: blank screen when app is closing due to external signals
- fix: improve polkit rule to let user do wifi scan

- feat: add `sensors` service with temperature and light
- feat: show temperature and light values in the footer
- refactor: organize order of loading of services

- refactor: housekeeping - better organize `/ubo_app/store` in directories
- feat: automatically run update check whenever about menu is opened with a throttling of 10 seconds

- feat: introduce notification ids as a means to avoid duplicated notifications
- fix: don't show expired notifications
- feat: add a convention for notification ids and status icon ids in `README.me`