[Clipboard] setData and getData implementations added

[pkosko]

Clipboard.getData, Clipboard.setData implementation part of https://github.sec.samsung.net/f-project/home/issues/35
This PR adds naive implementation using no platform specific APIs to share system clipboard. Because of internal issues of CBHM API, which causes not deterministic behaviour of API flow and missing native support on non-mobile profiles I decided to provide simple implementation for only sharing some clipboard data inside application.

As Clipboard.hasStrings() API is not described in docs https://api.flutter.dev/flutter/services/Clipboard-class.html, I removed part for handling such message.

[Before] Clipboard.setData and Clipboard.getData features were not implemented

[After] Naive implementation of clipboard shared only inside Flutter application
and only via Clipboard API. Such decision was done because of supporting partial
functionality on all profiles. CBHM API is supported only on mobile and currently
it is not possible to support it in other way.

Dart code below is able to set/get internal clipboard data:

      ClipboardData cd = ClipboardData(text: 'some text');
      Clipboard.setData(cd);

      Future<ClipboardData?> d = Clipboard.getData(Clipboard.kTextPlain);
      void dataCallback(ClipboardData? d) {
        if (d != null) {
          String? text = d.text;
          if(text != null) {
            print("Clipboard data $text");
          }
        }
      }
      d.then(dataCallback);

[rwalczyna]

Please rebase.