Fixed the Network category that was not visible
An appendRow() was needed
Store all items on the same list and make the model flat

Storing all items into the same list makes things easier.
We'll tell which type an item is by casting it. This is not always
possible now because PlacesItem is used for almost every item type
rather being used as a generic type.

We also make the model flat because QtQuick doesn't have any tree view
therefore we need to separate different items with sections on the
"category" property.

In my design each item type implementation will set the "category"
property to a translatable string (such as Places, Bookmarks, etc...).
Store all items on the same list and make the model flat

Storing all items into the same list makes things easier.
We'll tell which type an item is by casting it. This is not always
possible now because PlacesItem is used for almost every item type
rather being used as a generic type.

We also make the model flat because QtQuick doesn't have any tree view
therefore we need to separate different items with sections on the
"category" property.

In my design each item type implementation will set the "category"
property to a translatable string (such as Places, Bookmarks, etc...).
Rename Bookmark to BookmarkItem

This makes it more easy to understand what the class does.
Static bookmarks

Reorganize static bookmarks.
Set icon size to 22x22

Icon size is static for now.
Fix warning about members order

Properties order must be respected.
Rename Volume and Mount to VolumeItem and MountItem

Rename these classes to avoid conflicts with Kommodity.
Merge MountItem into VolumeItem

There's no need to make a difference. Kommodity's Mount are just Volumes
which are mounted and we can always get a pointer to each other.
Fixed wrong "File System" icon name

Follow icon naming specification using drive-harddisk instead of
volume.
Long live FolderModel and FolderItem

Add a folder model implementation using Kommodity.
Handle volumes and mounts added, changed or removed

React when volumes and mounts are added, changed or removed.
Highlight selected item

Draw a rectangle over the selected item and change text color.
When an item is selected we also emit a signal with the selected URI.
Network items after devices

Move the "Network" category items after "Devices".
Improve volume items handling

Known volumes are now added to the items list and new volumes don't
duplicated the "Devices" category.
Method isMounted() has been fixed
Remove useless PlacesModel::itemFromBookmark() method

It's not useful and don't get what it should do.