color-selection: Take hex codes that don't start with "#"

Previously if a user input a valid hex code, but left out
"#", `gdk_rgba_parse ()` would fail.

This commit ensures that we prepend "#" before parsing, so
that codes without it are still handled properly.

Closes #109
3 jobs for wip/christopherdavis/easy-hex-input in 1 minute and 22 seconds (queued for 2 seconds)
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flatpak

00:00:32

 
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review

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passed #487526
manual
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