Gtk-4: Add initial suppport to gtk v4

gtk-4.0/apps/_geary.scss

@@ -0,0 +1,37 @@
+/*********
+ * Geary *
+ *********/
+
+.geary-titlebar-left,
+.geary-titlebar-right {
+
+  .separator {
+    opacity: 0;
+  }
+}
+
+ConversationListView {
+  -GtkTreeView-grid-line-width: 0;
+
+  .view {
+    &:active,
+    &:selected {
+      background-color: $selected_bg_color;
+      color: $selected_fg_color;
+
+      &:backdrop {
+        background-color: $backdrop_selected_bg_color;
+        color: $backdrop_selected_fg_color;
+      }
+    }
+
+    .cell {
+      border: solid transparentize(black, 0.8);
+      border-width: 0 0 1px 0;
+      &:selected {
+        color: $selected_fg_color;
+        border: 0px solid darken($selected_bg_color, 10% );
+      }
+    }
+  }
+}