color-settings: Destroy dialog before releasing its GtkBuilder

This amends 4bc8ef84, where the dialog
destruction was improperly removed. Although this is of no consequence
in this case, since the dialog is identified with the main program, it
is the user's responsibility to destroy toplevel windows from a
GtkBuilder, according to the documentation and contrary to what was said
in that commit.

Related: !65
MR: !70
2 jobs for !70 with color-settings-release in 2 minutes and 53 seconds (queued for 2 seconds)
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