Add categories field to WebAppMeta

It defaults to "Network;" with a warning.

Spec: http://standards.freedesktop.org/menu-spec/latest/apa.html

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Add categories to web app desktop files

They will be shown in a proper applications submenu.

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Add AudioVideo;Audio; categories to test service

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Add info about categories to doc

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>