Initialize the app with popular folders

[kirillt]

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

[kirillt]

Additional requirements:

• folder suggesting dialog must be separate from scan dialog
• folder suggesting dialog must be presented after scan dialog
• if any of suggested folders is already added, it should be excluded from suggestions
• if no folders remain to suggest, folder suggesting dialog can be omitted

[kirillt]

Also, improvements to scan dialog can be made:

• if the app can't find .ark-global folder with a list of roots inside, it should ask for scan
• if there is .ark-global folder with a list of roots inside, the app shouldn't ask for scan