Fix Bug 331156 - Display of HTML-Message extremely slow

FIXED-IN: 4.13.3
BUG: 331156

It's not a fix because it's in webkit the problem and there will not new update of webkit.
I added a variable to disable/enable htmlquote (it's disable by default).
=> we will lose html quoted color :(
Too bad.
I will add a gui in 4.14 to configure it.