[Improvement] Customization improvements for widgets/graph layout management

[timdegroot1996]

A collection of customization improvements that could make dashboard layout management more flexible:

• Add empty graph/widget placeholders to preserve vertical spacing when widgets are disabled (currently widgets always move up).
• Add a dedicated Stat Widget and move "Add Custom" options to the section header instead of showing them as dashboard blocks (also applicable to the URL Widget feature).
• Add "Add All Widgets" functionality (per section or globally), with optional random or user-selected colors.
• Add a "Move to Top" action for individual widgets/graphs.
• Add a "Move to Bottom" action for individual widgets/graphs.
• Support multi-select and drag-and-drop for widgets (if supported by GridStack).