Also add a slideshow test script

This was moved to systemd

Note: This commit breaks the slideshow. This early in the cycle we just need
the installer to run and these dependencies prevent building the ISO. We'll
fix that in a later commit.

Add optional /@ and /@home subvolume mount-targets for btrfs (to satisfy timeshift btrfs snapshot feature)

When asked the length of a partition, Parted sometimes returns a negative
value.

df is more accurate and more reliable.

This commit prefers the df value when assessing whether or not to
skip partitions which are less than 5MB.

- At the end of the installation, the installer could quit before
showing the restart dialog.

- In expert mode, the installer could continue instead of waiting
on the pause screen.

Although the script worked with /dev/sr0, it didn't work say with
/dev/sdc1. In that case it needs to check in sys/block
for sdc, as opposed to sdc1.

Translate button labels explicitly.

- Explain in the UI why/when en_US is added
- Don't add en_US if variant is Latin
- Use ctrls_toggle as switch shortcut (alt+shift conflicts with shift+alt+tab)
- Fix variant selection
- Add more non-latin layouts to the list (serbian etc..)
- Fix keyboard preview generation

These items didn't change when a different language was selected.

Attempts to fix https://github.com/linuxmint/lmde-3-cinnamon-beta/issues/13.

mkfs.btrfs is in /bin (not /sbin), so it wasn't detected.

While fixing this, I tried to use a ListStore(str) model (i.e.
with only one column). For some reason the combobox kept showing
two columns instead of one, both with identical values.

Using two columns in the model and setting one to "" worked around
the issue.

The Cancel/OK buttons were not working.
The response wasn't read from the dialog, so changes were
applied when the dialog was closed.

It makes it really tedious to work on live-installer :)

Otherwise it fails at partitioning stage

This avoids duplication and eases the maintenance of the list.

Additional US layout and toggle hotkey for choosed non-latin layouts in target system

Consistent with Mint and Calamares.