Add bookmarks for pictures, music and videos

BUG: 427876 (https://bugs.kde.org/show_bug.cgi?id=427876)

This is a proposal, feel free to object. There has been a discussion about this before, so adding for reference: https://phabricator.kde.org/D10245.

Reasoning from my side: I am used to access folders on my home directory rather often, especially when it comes for Screenshots taken with Spectacle that are stored by default in the Pictures folder. Thus I added the common folders. I liked how it works with GNOME or Windows.

To test you have to remove (or temporary rename, move etc.) ~/.local/share/user-places.xbel and its backup files.

Screenshot: