Replace inline-styles by external CSS

Selected inline-styles were externalized to xterm.css.

The `Terminal._charSizeStyleElement` is removed since it
was not used anymore.

Fixes: https://github.com/xtermjs/xterm.js/issues/1335

src/xterm.css

@@ -117,11 +117,13 @@
     visibility: hidden;
 }
 
-.xterm .xterm-char-measure-element {
+.xterm-char-measure-element {
     display: inline-block;
     visibility: hidden;
     position: absolute;
+    top: 0;
     left: -9999em;
+    line-height: normal;
 }
 
 .xterm.enable-mouse-events {
@@ -151,3 +153,7 @@
     height: 1px;
     overflow: hidden;
 }
+
+.xterm-cursor-pointer {
+    cursor: pointer;
+}