Fix an issue with the `class` attribute in the tree view and replace an ampersand by the corresponding character entity

- Commit 79e1033549b922089ccd75bc3922f6c8500267a4: Fix the anchor `class` attribute in the tree view
- Commit 4b26ae26a0eb8390c19a4d045993f0c540e78643: Fix an unescaped ampersand in the `PageTree` widget