UI: implement 'pumblend' option for semi-transparent popupmenu

Why?

- Because we can.
- Because the TUI is just another GUI™
- Because it looks kinda nice, and provides useful context like 1 out of 100
  times

Complies with "don't pay for what you don't use".

Some crashes for resizing were unfolded, add tests for those.

test/functional/ui/screen.lua

@@ -1303,6 +1303,8 @@ function Screen:_pprint_attrs(attrs)
       if f == "foreground" or f == "background" or f == "special" then
         if Screen.colornames[v] ~= nil then
           desc = "Screen.colors."..Screen.colornames[v]
+        else
+          desc = string.format("tonumber('0x%06x')",v)
         end
       end
       table.insert(items, f.." = "..desc)