[gtk viwer] Drop deprecated function

No idea why it was needed, lets hope this is OK.
[gtk widget] Disable useless deprecation warning

At least until there is a non-deprecated way to get cursor color.
[gtk widget] Show cursor even without cursor-color
[gtk widget] Fix cursor width calculation
[gtk widget] Fix selection drawing with recent GTK

Without adding a CSS class we get no background color.
Minor
[gtk widget] Simplify

We want highlight selection color even if the window is not active.