gears.color: Add transparent constant

Avoid repetitive "#00000000" for transparent wiboxes and such
elements.

lib/gears/color.lua

@@ -333,6 +333,9 @@ end
 
 pattern_cache = require("gears.cache").new(color.create_pattern_uncached)
 
+--- No color
+color.transparent = color.create_pattern("#00000000")
+
 return setmetatable(color, color.mt)
 
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