Design settings activity/UI

Build UI with XML
Build activity to load XML
Add strings
Modify Manifest to declare the activity
Modify MainActivty to launch the PreferenceActivity
Load the preferences for the calculs

Add a dedicated method to retreive preference from SharedPreference and
load them in the Settings class (will be removed later) for the calcul.
This method is called only while launching the calcul.
Merge remote-tracking branch 'origin/Dev_Settings'