feat(tabs): add automatic scrolling when holding down paginator (#14632)

Adds some code that automatically keeps scrolling the tab header while holding down one of the paginator buttons. This is useful on long lists of tabs where the user might have to click a lot to reach the tab that they want.

Fixes #6510.

tools/public_api_guard/lib/tabs.d.ts

@@ -109,11 +109,13 @@ export declare class MatTabGroupBase {
     constructor(_elementRef: ElementRef);
 }
 
-export declare class MatTabHeader extends _MatTabHeaderMixinBase implements AfterContentChecked, AfterContentInit, OnDestroy, CanDisableRipple {
+export declare class MatTabHeader extends _MatTabHeaderMixinBase implements AfterContentChecked, AfterContentInit, AfterViewInit, OnDestroy, CanDisableRipple {
     _disableScrollAfter: boolean;
     _disableScrollBefore: boolean;
     _inkBar: MatInkBar;
     _labelWrappers: QueryList<MatTabLabelWrapper>;
+    _nextPaginator: ElementRef<HTMLElement>;
+    _previousPaginator: ElementRef<HTMLElement>;
     _showPaginationControls: boolean;
     _tabList: ElementRef;
     _tabListContainer: ElementRef;
@@ -129,14 +131,21 @@ export declare class MatTabHeader extends _MatTabHeaderMixinBase implements Afte
     _getLayoutDirection(): Direction;
     _getMaxScrollDistance(): number;
     _handleKeydown(event: KeyboardEvent): void;
+    _handlePaginatorClick(direction: ScrollDirection): void;
+    _handlePaginatorPress(direction: ScrollDirection): void;
     _isValidIndex(index: number): boolean;
     _onContentChanges(): void;
-    _scrollHeader(scrollDir: ScrollDirection): void;
+    _scrollHeader(direction: ScrollDirection): {
+        maxScrollDistance: number;
+        distance: number;
+    };
     _scrollToLabel(labelIndex: number): void;
     _setTabFocus(tabIndex: number): void;
+    _stopInterval(): void;
     _updateTabScrollPosition(): void;
     ngAfterContentChecked(): void;
     ngAfterContentInit(): void;
+    ngAfterViewInit(): void;
     ngOnDestroy(): void;
     updatePagination(): void;
 }