Append "Network" category

It was not visible because the root item was not appended.
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.
Fix wrong icon name for "File System"

Follow icon naming specification and use drive-harddisk rather than
volume.
Now class FileInfo's method getIcon returns QIcon
Better implementation is needed
Add FolderModel and FolderItem

This implements a model for folder contents and its items 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".
Correct volumes and mounts in PlacesModel

Known volumes are now appended to m_items so that they can be found
later.  New volumes are appended to the "Devices" category instead of
duplicating it.
Method isMounted() has been fixed

Don't cast pointer to boolean.
Remove useless PlacesModel::itemFromBookmark() method

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