Welcome modal (greeter)

**Is your feature request related to a problem? Please describe.**
When the user opens the Frank!Flow (for the first time), the user gets to see an empty file (or last edited if possible).

**Describe the solution you'd like**
The experience could be better if the user was presented with a window, asking which action should be made.
These actions could contain:

- Open last edited adapter
- Create a new adapter in existing configuration
- Create a new configuration

Features for these actions should be made when the MVP is done.

**Describe alternatives you've considered**
Alternatives can be discussed.