Use dates in Contents to be able to cache folders in server

Use-cases to test that I think of:

• first run (no gcompris-qt.conf and no ~/.cache/KDE/gcompris-qt/ folders)

• accept -> voices + background music + wordset (there is actually a bug with wordset not being loaded at first run)

• refuse -> nothing downloaded. Either check or uncheck automatic downloads

• second run with refused at first run and automatic downloads (gcompris-qt.conf and no ~/.cache/KDE/gcompris-qt/ folders)

• will download voices + background music + wordset

• second run with refused at first run and no automatic downloads (gcompris-qt.conf and no ~/.cache/KDE/gcompris-qt/ folders)

• will not download voices + background music + wordset

Use-cases with full rcc :

• first run and full.rcc exists (gcompris-qt.conf does not exist)

• if answer no to download, don't download anything and use full.rcc
• if answer yes to download, download and use separate rcc files, and keep full.rcc as fallback

• second run and full.rcc exists and automatic (gcompris-qt.conf does exist)

• download/update separate voices + background music + wordset / voices and wordset present

• No network:

• Manually adding rcc/Contents files without network at all.

• first run with separate rcc files already installed (in user or system path):

• It should show the dialog asking if user wants to automatically download/update files (of course only if WITH_DOWNLOAD=ON). Now it doesn't show the dialog and defaults to automatic downloads.

• RCC files installed in system path:

• Use rcc files in system path if they exist (all same behaviour as rcc files in user path, except don't try to delete them when downloading new ones).
• If newer files exist on the server, download them in user path and use them instead of system files.

• WITH_DOWNLOAD flag:

• Adding rcc/contents in system path (for embedded apk, and some linux distros packaging it, on linux the path would be /usr/share/KDE/gcompris-qt/data3/). And with or without the WITH_DOWNLOAD build flag: with WITH_DOWNLOAD=OFF use the system rcc files and all download features are disabled; with WITH_DOWNLOAD=ON check if newer files exist on the server and install them in ~/.cache/KDE/gcompris-qt/data3/ if automatic download is activated or when pressing the button to check for updates, else use system rcc files if they are up-to-date.

• Misc:

• Make sure that the build flag DOWNLOAD_ASSETS still works as it should (used in android_build_embedded_apk.sh for example).

• Need to check if we clean the previous rcc and if we do, how