#333 Make separation bars in mailbox display an option

[Anonymous]

When updating from 1.2.x to 1.4.2 I recognized that
the appearance of the mailbox display changed:
between the lines for each message, a small
separating line in the color of the headline is
displayed in 1.4 which wasn't there in 1.2.
As I found these lines irritating (I use alternative
colors for the rows anyway) I searched for an option
to switch this off. As this is not possible in 1.4.2,
I wrote this little patch which makes that an option.

[Nobody/Anonymous]

PATCH

[Thijs Kinkhorst]

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Thank you for your patch. This might be useful for some
users but we won't be implementing this: this is something
for when we have templating.