(Feature Request) Add startup page to set root directory on initial startup

[ner216]

Proposal:
To improve ease-of-use for new users, it may be beneficial to add a startup page that only appears if the root folder is not set (on initial startup), and ask the user to select a root folder. It could also give the user a hint as to where to change the root folder in the preferences.

[mkay]

There was already a welcome page - welcome.py, an AdwStatusPage with a "Set a Root Directory" pill button, shown when no usable root was set. But you probably just never got to see it because
a) of the crash from PR #2 happened before the window was shown, and
b) even without it, it was only shown if ~/Documents (as default/fallback) didn't exist. And ~/Documents exists on most systems - just not mine :)

I have fixed this slightly egocentric choice by removing a default. (~/Documents being a bad choice for many reasons)
Now a fresh install starts with no root folder at all and the welcome page asks you to pick one.
Thanks for the nudge.