Control the number of decimals in the ctkMatrixWidget display

Display the matrix values with the same number of decimals used to edit the
items.
Merge branch 'ctkdoublespinbox-decimalsbykey'

* ctkdoublespinbox-decimalsbykey:
  Control the number of decimals in the ctkMatrixWidget display
Expose publicly setDecimals and decimalsChanged
Merge branch 'ctkdoublespinbox-decimalsbykey'

* ctkdoublespinbox-decimalsbykey:
  Expose publicly setDecimals and decimalsChanged