Draft: Issue #3338751: Grid layout groups/modifiers UI improvement

Closes #3338751.

modules/paragraphs_collection_demo/paragraphs_collection_demo.paragraphs.grid_layouts.yml

@@ -13,6 +13,7 @@ paragraphs_collection_demo_equal_columns:
 
 
 paragraphs_collection_demo_1_1_column:
+  group: '2 Columns'
   title: 'Two columns 1 - 1'
   description: 'Two equal column layout.'
   wrapper_classes:
@@ -28,6 +29,7 @@ paragraphs_collection_demo_1_1_column:
     - 'paragraphs_collection_demo/grid_layout'
 
 paragraphs_collection_demo_1_2_column:
+  group: '2 Columns'
   title: 'Two columns 1 - 2'
   description: 'Two columns layout of 1/3 and 2/3 width.'
   wrapper_classes:
@@ -43,6 +45,7 @@ paragraphs_collection_demo_1_2_column:
     - 'paragraphs_collection_demo/grid_layout'
 
 paragraphs_collection_demo_2_1_column:
+  group: '2 Columns'
   title: 'Two columns 2 - 1'
   description: 'Two columns layout of 2/3 and 1/3 width.'
   wrapper_classes:
@@ -58,6 +61,7 @@ paragraphs_collection_demo_2_1_column:
     - 'paragraphs_collection_demo/grid_layout'
 
 paragraphs_collection_demo_1_3_column:
+  group: '2 Columns'
   title: 'Two columns 1 - 3'
   description: 'Two columns layout of 1/4 and 3/4 width.'
   wrapper_classes:
@@ -73,6 +77,7 @@ paragraphs_collection_demo_1_3_column:
     - 'paragraphs_collection_demo/grid_layout'
 
 paragraphs_collection_demo_3_1_column:
+  group: '2 Columns'
   title: 'Two columns 3 - 1'
   description: 'Defines a two columns layout of 3/4 and 1/4 width.'
   wrapper_classes:
@@ -88,6 +93,7 @@ paragraphs_collection_demo_3_1_column:
     - 'paragraphs_collection_demo/grid_layout'
 
 paragraphs_collection_demo_1_2_1_column:
+  group: '3 Columns'
   title: 'Three columns 1 - 2 - 1'
   description: 'Three columns layout of 1/4, 1/2 and 1/4 width.'
   wrapper_classes:
@@ -106,6 +112,7 @@ paragraphs_collection_demo_1_2_1_column:
     - 'paragraphs_collection_demo/grid_layout'
 
 paragraphs_collection_demo_2_1_1_column:
+  group: '3 Columns'
   title: 'Three columns 2 - 1 - 1'
   description: 'Three columns layout of 1/2, 1/4 and 1/4 width.'
   wrapper_classes:
@@ -124,6 +131,7 @@ paragraphs_collection_demo_2_1_1_column:
     - 'paragraphs_collection_demo/grid_layout'
 
 paragraphs_collection_demo_1_1_2_column:
+  group: '3 Columns'
   title: 'Three columns 1 - 1 - 2'
   description: 'Three columns layout of 1/4, 1/4 and 1/2 width.'
   wrapper_classes: