Changed TitleBar.uss to use class names rather than naming

I think we need some proper guidelines on USS usage. I think a sensible
rule would be that names are reserved for the owning view, such that it
can be used for layouting etc.

MaterialGraphProject/Assets/GraphFramework/SerializableGraph/Editor/Drawing/Styles/TitleBar.uss

-#TitleBar {
+TitleBarDrawer {
     flex-direction: row;
     justify-content: space-between;
     align-items: stretch;
 }
 
-#TitleBar #left,
-#TitleBar #right {
+TitleBarDrawer #left,
+TitleBarDrawer #right {
     flex: 1;
     height: 16;
     background-color: #3d3d3d;
 
-#TitleBar #left {
+TitleBarDrawer #left {
-#TitleBar #right {
+TitleBarDrawer #right {
     justify-content: flex-end;
 }