main/postmarketos-ui-sway: modernize

• use tinydm instead of lightdm with autologin (Much more lightweight; result is the same, you get automatically logged in and it's more consistent with other UIs. If somebody wants to add a password prompt, I suggest using FDE or simply not using postmarketos-ui-sway.)
• set default term to foot, as alacritty needs a recent opengl version (fix #984 (closed))
• override sway config to actually use foot (there is no sway-sensible-terminal and upstream isn't interested in adding it)
• set the pmOS wallpaper in the overridden sway config
• change $mod to Alt, so it's easier for me to test this in QEMU while running i3/sway on the host with $mod set to Mod4. This is only done for qemu now via subpackage, default is still mod4.
• re-enable previously disabled architectures
• set myself as maintainer (@danct12, feel free to protest/review)