EMSUSD-877 UI tweaks

- Move add-layer button left to the border of the panel.
- Don't show context-menu mouse cursor over action buttons.
- Make hover highlight even over muted rows.
- Make hover highlight less dark.
- Rename some paint context flag to be clearer.
- Move "USD Stage" label to the left border.
- Don't draw text under icons with hovering.
- Make text lighter when selected.
- Simplify and centralize the action icon rectangle calculations.
- Improve arrow appearance: smaller, more square, in 6x6 square.
- Adjustment of spacing.
- Remove unnecessary error log when a layer cannot be found. The UI
  already shows a warning icon.

lib/usd/ui/layerEditor/layerEditorWidget.cpp

@@ -103,9 +103,8 @@ LayerEditorWidget::LayerEditorWidget(SessionState& in_sessionState, QMainWindow*
 QLayout* LayerEditorWidget::setupLayout_toolbar()
 {
     auto buttonSize = DPIScale(24);
-    auto margin = DPIScale(12);
     auto toolbar = new QHBoxLayout();
-    toolbar->setContentsMargins(margin, 0, 0, 0);
+    toolbar->setContentsMargins(0, 0, 0, 0);
     auto buttonAlignment = Qt::AlignLeft | Qt::AlignRight;
 
     auto addHIGButton

lib/usd/ui/layerEditor/layerTreeItem.cpp

@@ -140,37 +140,15 @@ void LayerTreeItem::populateChildren(RecursionDetector* recursionDetector)
     for (auto const path : subPaths) {
         std::string actualPath = SdfComputeAssetPathRelativeToLayer(_layer, path);
         auto        subLayer = SdfLayer::FindOrOpen(actualPath);
-        if (subLayer) {
-            if (recursionDetector->contains(subLayer->GetRealPath())) {
-                MString msg;
-                msg.format(
-                    StringResources::getAsMString(StringResources::kErrorRecursionDetected),
-                    subLayer->GetRealPath().c_str());
-                puts(msg.asChar());
-            } else {
-                auto item = new LayerTreeItem(
-                    subLayer,
-                    LayerType::SubLayer,
-                    path,
-                    &_incomingLayers,
-                    _isSharedStage,
-                    &_sharedLayers,
-                    recursionDetector);
-                appendRow(item);
-            }
-        } else {
-            MString msg;
-            msg.format(
-                StringResources::getAsMString(StringResources::kErrorDidNotFind),
-                std::string(path).c_str());
-            puts(msg.asChar());
+        if (!subLayer || !recursionDetector->contains(subLayer->GetRealPath())) {
             auto item = new LayerTreeItem(
                 subLayer,
                 LayerType::SubLayer,
                 path,
                 &_incomingLayers,
                 _isSharedStage,
-                &_sharedLayers);
+                &_sharedLayers,
+                recursionDetector);
             appendRow(item);
         }
     }

lib/usd/ui/layerEditor/layerTreeItem.h

@@ -107,6 +107,12 @@ class LayerTreeItem : public QStandardItem
     // refresh our data from the USD Layer
     void fetchData(RebuildChildren in_rebuild, RecursionDetector* in_recursionDetector = nullptr);
 
+    enum Roles
+    {
+        // Data role containing a boolean indicating if the mouse is over an action button.
+        kHoverActionRole = Qt::UserRole + 1
+    };
+
     // QStandardItem API
     int      type() const override;
     QVariant data(int role) const override;