Add webkit-prefixed user-select attributes (#41)

It turns out that WebKit uses its own prefix for user-select so I'm adding this everywhere we currently define user-select, as well as a few new places

frontend/app/workspace/workspace.less

@@ -46,8 +46,6 @@
     height: 32px;
     border-bottom: 1px solid var(--border-color);
     flex-shrink: 0;
-    user-select: none;
-    cursor: default;
 
     .tab {
         display: flex;
@@ -57,6 +55,8 @@
         height: 100%;
         font-weight: bold;
         border-right: 1px solid var(--border-color);
+        user-select: none;
+        -webkit-user-select: none;
         cursor: pointer;
         position: relative;