Initial styling for new room list

This is a work in progress, but covers the coarse areas. This uses all-new classes to better describe what everything is, and to reduce the number of selectors we keep track of.

This is primarily layout for the list and not actually the final structure. For example, some buttons are missing and other areas are not styled correctly - the idea in this commit was to get things roughly in the right place and work on it.

res/css/structures/_MatrixChat.scss

@@ -66,7 +66,7 @@ limitations under the License.
 }
 
 /* not the left panel, and not the resize handle, so the roomview/groupview/... */
-.mx_MatrixChat > :not(.mx_LeftPanel_container):not(.mx_ResizeHandle) {
+.mx_MatrixChat > :not(.mx_LeftPanel_container):not(.mx_LeftPanel2):not(.mx_ResizeHandle) {
     background-color: $primary-bg-color;
 
     flex: 1 1 0;