Add underlayer / overlayer for the selection and cursor

The renderedLines div can now only contain actual lines, nothing else. Thes divs are designed to match the lines div in dimensions and make it easy to put things in front of or behind the text.

static/editor.css

@@ -73,6 +73,17 @@
   overflow-x: hidden;
 }
 
+.editor .underlayer, .editor .overlayer {
+  pointer-events: none;
+  position: absolute;
+  width: 100%;
+  height: 100%;
+}
+
+.editor .underlayer {
+  z-index: 0;
+}
+
 .editor .lines {
   position: relative;
   display: table;
@@ -80,8 +91,14 @@
   width: 100%;
   /*overflow: hidden; i'm worried this is causing rendering problems */
   padding-right: 2em;
+  z-index: 1;
 }
 
+.editor .overlayer {
+  z-index: 2;
+}
+
+
 .editor .line span {
   vertical-align: top;
 }