Add placeholder if transactions list is empty

Adds a placeholder for the transactions list recycler view if no transactions are present to reduce blankness of initial screen.

Tested with:

  • New wallet, both in english and germane locale
  • Placeholder disappears after first transaction is received
  • Doesn't overlap with fab buttons
  • Slides together with collapsing toolbar so that once collapsed it sits in the middle of the swipe container

If another image is preferred I'm open to try my best and re-design...

Screenshot_placeholder

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