Only accept numbers in FactTextEdit

All facts are numeric, atm. this means that the input should only
accept numbers. it already had the     inputMethodHints:   Qt.ImhFormattedNumbersOnly
option, but for some reason it still accepted letters and symbols.
by adding a     validator: DoubleValidator {} we limit the characters
to numbers only - and if the value is a integer, it will still work.

Tested by adding waypoints on the map & messing with the GPS coordinates.

Signed-off-by: Tomaz Canabrava <tomaz.canabrava@intel.com>