Add z-index to editor, add default styles for internal inline-tools (#566)

src/styles/block.css

@@ -61,4 +61,17 @@
       );
     }
   }
+
+  a {
+    cursor: pointer;
+    text-decoration: underline;
+  }
+
+  b {
+    font-weight: bold;
+  }
+
+  i {
+    font-style: italic;
+  }
 }

src/styles/ui.css

@@ -4,6 +4,7 @@
 .codex-editor {
   position: relative;
   box-sizing: border-box;
+  z-index: 1;
 
   .hide {
     display: none;