Initialize the app with popular folders

When a user opens the app for the very first time, empty folders list could be confusing.

We need to provide a user with option to initialize the app with default folders.
These folders could be:
* `DCIM/Camera`
* `Pictures`
* `Downloads`
* `Documents`

For this issue be considered solved, we need:
- [ ] determine clear list of "popular" folders
- [ ] determine variations of each path in different Android versions
- [ ] modify "first run dialog" in the way it would not only suggest to scan filesystem for previously created folders, but also to add "popular" folders