core: DownloadManager: add a generic resource path for manual copying

Instead of only one path for maintaining downloaded .rcc files we now
support a sorted list of local resource paths. In this list the known
target download path is always checked before the other ones for .rcc
files.

Besides the download path we also try to load .rcc files from
QStandardPaths::GenericDataLocation now, which is

  ... $HOME/.local/share/GCompris on linux desktop, or
  ... /storage/sdcard0/GCompris on Android.

This base-path can be used by the user to manually put rcc files, in
case he/she does not have an internet connection at all.

Also inlined some methods.