[Layout] Support sub-object selection in layout (#1811)

Updates to sub object selection, first cut of selection APIs.

* [API] Add inspector view registry to register inspector view providers and show a view in the inspector.

[API] Modify the selection API to register the click event and handle the event. The API will add a class to the selected object and the immediate parent of the selected object.

[Directive] Implemenet mct-selectable directive for making an element selectable.

[Layout] Update the layout controller to use the Selection API. Also, add double click gesture to allow drilling into a selected object.

Populate the Elements pool with contained elements of the selected object. Update toolbar and inspector to listen for the changes in selection.

* [Frontend] Mods to markup and CSS for sub-object selection

* MCTSelectable allows selection in initialization, use to select on navigation

[Frontend] Show grid in first nested layout, hide from deeper nesting. Only show grids when applicable to relative selection.

* Fix checkstyle and lint errors

* Bring back the change that made mct-init-select work

* [Inspector] Make sure the right content is displayed based on whether a view provider exists or not.

* Only show table options when editing

* Make reviewers' requested changes

* Fix broken tests

* [Frontend] Cleanups and tweaks

Fixes #1811
- Cleanups between frame, editor and selecting.scss;
- Hover and selected borders visually pumped up a bit;
- Solid borders on hover and selecting when browsing;
- Dashed borders for layouts when editing;
- Fixed cursor to only show move capability when
element is selected;

* [Frontend] Tweaks to frame.no-frame layout

Fixes #1811
- Margin set to 0;
- Overflow set to hidden;

* [Frontend] Fixed position items border width fixed

Fixes #1811
- Set to 1px;

* Add tests for inspector controller and fix broken tests. Clean up code.

* [Fixed Position] Stop event propagation on click handlers in fixed position to avoid the event reaching the selection click handlers which caused issues with toolbar and selection."

* Fix tests

* Add tests

* Add test

* Remove element from document

platform/commonUI/general/res/sass/edit/_editor.scss

@@ -80,23 +80,32 @@
 
     // Editing Grids
     .l-grid-holder {
-        display: block;
         .l-grid {
             &.l-grid-x { @include bgTicks($colorGridLines, 'x'); }
             &.l-grid-y { @include bgTicks($colorGridLines, 'y'); }
         }
     }
 
-    // Prevent nested frames from showing their grids
-    .t-frame-outer .l-grid-holder { display: none !important; }
-
-    // Prevent nested elements from showing s-hover-border
-    .t-frame-outer .s-hover-border {
-        border: none !important;
+    // Display grid when selected or selection parent.
+    .s-selected .l-grid-holder,
+    .s-selected-parent .l-grid-holder {
+        display: block;
     }
 
-    // Prevent nested frames from being selectable until we have proper sub-object editing
-    .t-frame-outer .t-frame-outer {
-        pointer-events: none;
+    // Display in nested frames...
+    .t-frame-outer {
+        // ...when drilled in or selection parent...
+        &.s-drilled-in, &.s-selected-parent {
+            .l-grid-holder {
+                display: block;
+            }
+            .t-frame-outer:not(.s-drilled-in) .l-grid-holder {
+                display: none;
+            }
+        }
+        // ...but hide otherwise.
+        .l-grid-holder {
+            display: none;
+        }
     }
 }