Show UI selection ordered by name, `none` remains at the beginning

I've noticed that the UI selection order was different every execution of pmbootstrap init. This was due the usage of a dictionary which doesn't maintain the order. Since pmb.helpers.ui.list(args) function returned this dictionary it was possible to order it later by the keys but this was showing the none ui in the middle of the list. I changed the pmb.helpers.ui.list(args) function to return a list of tuples instead, so that the order of the data inserted is maintained. In the ask_for_ui(args) function I had to change the way to validate if the user input is valid, and the simplest way is to convert the list of tuples to a dictionary and get the list of keys from it, and just check if the user input is in this list.

Result:

[23:10:42] Available user interfaces (6):
[23:10:42] * none: No graphical environment
[23:10:42] * hildon: (X11) Lightweight GTK+2 UI (optimized for single-touch touchscreens)
[23:10:42] * luna: (Wayland) webOS UI, ported from the LuneOS project (Not working yet)
[23:10:42] * mate: (X11) MATE Desktop Environment, fork of GNOME2 (stylus recommended)
[23:10:42] * plasma-mobile: (Wayland) Mobile variant of KDE Plasma, optimized for touchscreen
[23:10:42] * weston: (Wayland) Reference compositor (demo, not a phone interface)
[23:10:42] * xfce4: (X11) Lightweight GTK+2 desktop (stylus recommended)