Welcome screen

[philips77]

This PR adds a Welcome screen to nRF Mesh app.

User may select one of 3 initial network configurations:

1. Empty - only a random Primary Network Key (this is the legacy behavior)
2. Custom - user can quickly add required number of keys, groups and scenes
3. Debug - just like Custom, but the keys have non-random values: 0x0000...00nn, where nn is a UInt8 number, incremented by 1 for each key.
4. Import - allows importing previously exported network without first creating a dummy one.

[philips77]

E.g., if Network Keys is set to 3 and App Keys to 4 the wizard will create 3 network keys:

• 0000...0001
• 0000...0002
• 0000...0003

and 4 application keys:

• 0000...0004
• 0000...0005
• 0000...0006
• 0000...0007

all bound to the first (primary) network key. Bound keys can be switched in App key settings later.

[philips77]

And of course a disclaimer:

Warning
Don't use debug keys in production!