Display some help, instead of empty screen, when there are no entries

[pmartin]

When launching the application for the first time, there are no entries displayed (as there are no entries in DB and they are not fetched automatically)
=> Display something instead of this empty screen :

• How to configure the application with a JSON file (until there is a screen for that)
• How to launch a synchronization
• How to leave the application / open the menu
• If the screen is big enough : how to navigate, how to report problems (github + log file), ...

[pmartin]

Here's a first idea of what this might look like (based on current branch onboarding-help-when-no-local-data):

Of course, French translation will follow, once the idea seems OK ;-)