* Also use PKEXEC_UID to get username
Privilege elevation can occur through pkexec, where many environment
variables are not accessible.
This fixes the following problem - if howdy-gtk is run without sudo,
authorization occurs via pkexec, and the user variable is empty. So when
adding the first model, it is unclear to which user to add it.
* Update the container when the slide changes
In some GTK themes, when we change a slide, we see a blank window
because there is nothing to trigger the size update, and the slide is
displayed at zero size. Let's force a size update so that it always
works.
* Remove reading of non-existent '_variables' file
* More correct preview when stretching the window
The "Video" tab layout did not display correctly when resizing the
window.
* Don't add a model if the user list is empty
The list of users may be empty, and if you try to add a model, a string
concatenation error with None will occur.
For simplicity and consistency with the "Delete" button, we simply check
the size of the list after clicking.
* Show real camera ID in the 'Video' tab
* Handle the case if there are no cameras via except
Otherwise, when trying to read the /dev/v4l/by-path directory, an
exception is thrown and the program visually freezes.
