[Slicer] Integrated Slicer Header with Selection Summary (Count, Percentage Bar, and Reset)

[lcordovab]

As a business user it's critical to understand the context of my filters at a glance. Standard slicers don't indicate the scope of the selection.

**Business Problem:** When analyzing data with thousands of potential values in a dimension (e.g., Cost Centers, GL Accounts), selecting a subset leaves the user "blind." They can't easily see if they've selected 10 out of 100 or 10 out of 10,000, which is crucial context for analysis. We currently build complex workarounds with multiple visuals (Matrices, DAX, Bookmarks) to show this.

**Proposed Solution:** Enhance the Slicer visual to have an optional, integrated header section that can display:
1. **Selection Count:** A text summary of items selected vs. total available (e.g., "5688/9019").
2. **Proportional Bar:** A visual data bar showing the percentage of items selected.
3. **One-Click Reset:** A built-in "Reset" or "Select All" button within the slicer header, distinct from the main visual header's "Clear selections" eraser.

This would dramatically improve usability, reduce report development complexity, and provide essential analytical context directly within the primary filtering tool.