SD/CF dual slots free space display

[arnaud-sintes]

• lower "Temperature" UI info priority, so it may disappear when "Free space" took too much space
• add methods to get the cumulated size (GB) of the files in the DCIM folder of a given drive
• update free_space_update update function to display both CF & SD or a single card information depending if dual slots or not: in case of single slot we're displaying "SD:XXX" or "CF:XXX", in case of dual slots we're displaying "SD:XXX CF:XXX", "XXX" being the free space available in GB (unit is hidden)
  Note the number of slot is determined once (alongside the computation of the inital disk total size)