API docs: Improve code display.

This also repairs the text alignment in code lines from center to left.

Signed-off-by: Mavridis Philippe <mavridisf@gmail.com>

doc/common/doxygen.css

@@ -94,3 +94,16 @@ SPAN.comment       { color: #800000 }
 SPAN.preprocessor  { color: #806020 }
 SPAN.stringliteral { color: #002080 }
 SPAN.charliteral   { color: #008080 }
+
+.line {
+text-align: left;
+font-family: monospace;
+}
+
+.lineno {
+display: inline-block;
+width: 3em;
+margin-right: 5px;
+background: #EEEEEE;
+text-align: right;
+}