get rid of mbtiles and use local tiles folders instead
display tiles according to the connection status (local/remote)
Leaflet tilelayers url (xyz) can now be configured in app settings parameters, and is also used for device case
no more individual delete action on the list of treks
download zip and extract png for global background
do not unzip if file exists
remove all mbtiles related stuff
do not put files in storage root
download trek + poi + static in a single zip
in the preferences, be able to delete all folders and zip files for tiles having zoomlevel>12
Merge branch 'no_mbtiles' of github.com:makinacorpus/Geotrek-mobile into no_mbtiles

Conflicts:
	app/scripts/settings.js
Merge branch 'no_mbtiles'