fix(tabs): accessibility attributes are missing in Tabset component

[davidsekar]

Currently, the aria attributes are not added to Tabset component.
So I would like to contribute on this.

Please clarify following,

1. TabsModule requires unique ID to be generated for each and every tab, so that anchor tag with role='tab' can be linked to a panel with role='tabpanel'.
   Do we have a service or global variable, which helps to generate a unique id ?

2. If a new service can be used... Can a separate service be added to TabsModule ?
   Something like this to generate unique id

import { Injectable } from '@angular/core';

@Injectable()
export class ScrollableTabsService {
    private uniqueId: number;
    constructor() {
        this.uniqueId = 0;
    }

    get uniqueControlId(): number {
        let ret = this.uniqueId;
        this.uniqueId++;
        return ret;
    }
}

In tabset component

constructor(config: ScrollableTabsetConfig, scrollableTabsService: ScrollableTabsService) {
        this.controlId = scrollableTabsService.uniqueControlId;
        this.nextId = 0;
        Object.assign(this, config);
    }
...
addTab(tab: ScrollableTabDirective): void {
        this.tabs.push(tab);
        this.nextId++;
        tab.active = this.tabs.length === 1 && typeof tab.active === 'undefined';
    }

In tabs directive

constructor(
        tabset: ScrollableTabsetComponent,
        public elementRef: ElementRef,
        public renderer: Renderer2
    ) {
        this.tabset = tabset;
        this.id = 'scrollable-tab-' + this.tabset.controlId + '-' + this.tabset.nextId;
        this.ariaLabelledBy = this.id;
        this.panelId = this.id + '-panel';
        this.tabset.addTab(this);
    }

If the above approach is OK. I will raise a pull request.

Thanks for awesome project.

[yogsadafal]

@YevheniiaMazur any timeline for this?? looks pretty simple fix for accessibility.