Work started and tested in #188 : working

Requirements : 
- #218 for OrangeFox support (especially `fastboot_flash_recovery` and
`fastboot_reboot_recovery`)
- #220 for additional partitions support
- #221 for device specific notes (optional)

This PR is based on #220 (by @anon1892) and aims to integrate the code
in the Installer.

- Include the commands `fastboot_flash_recovery` and
`fastboot_reboot_recovery` (thanks to @anon1892)
- Add support for Mi439 (Redmi 7A & co.) (thanks to @anon1892)
- Display device specific notes at the image selection step (thanks to
@anon1892)

Other fixes & additions:
- Display the scroll bar by default if scrolling is necessary
- An info text box for additional images

Todo:
- [x] When the "custom recovery already flashed" button is selected,
user should not have to select recovery file, as in this PR for now.
- [x] User should be able not to select additional partitions, even on
Android 13 (maybe add a button like "custom recovery already flashed".
- [x] When recovery flashing is disabled, `adb reboot recovery` is not
performed. Fix that.