Show animated loading screen when first loading notes

[mirka]

After logging in to an existing account, the app displays a "No Notes" placeholder for a second while first loading all the notes.

It would save the users from a moment of panic if it were a loading screen instead.

[roundhill]

The native app shows a little spinner in the toolbar while it is indexing, it usually goes away in a second or two, but it is good to see that it's working on something :)

[codebykat]

We do now have a catch to show a "Loading" placeholder instead of "No Notes". #1649