Replace overlayDir.attach in MatSelectSearchComponent

[evoltafreak]

Replace overlayDir.attach in MatSelectSearchComponent with openedChange due deprecation in Angular 10.

fixes #208

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[macjohnny]

@evoltafreak I noticed a problem with this PR when using <ngx-mat-select-search> without a wrapping <mat-option> element:

    <mat-select [formControl]="bankCtrl" placeholder="Bank" #singleSelect>
      <!-- <mat-option> -->
        <ngx-mat-select-search [formControl]="bankFilterCtrl"></ngx-mat-select-search>
      <!-- </mat-option> -->

      <mat-option *ngFor="let bank of filteredBanks | async" [value]="bank">
        {{bank.name}}
      </mat-option>
    </mat-select>

when clicking the mat select the first time, the panel will jump after the animation, see e.g.
https://stackblitz.com/edit/github-3utpdn?file=src%2Fapp%2Fexamples%2F01-single-selection-example%2Fsingle-selection-example.component.html

since this use case is the only reason this part of the code exists, I would suggest to deprecate this use case and remove this code when angular 10 is released. what do you think?

[evoltafreak]

@macjohnny Yeah sure. I just wanted to mention the issue, so you won't be surprised when the property will be deprecated. Thanks for the testing.