Currently, if the user checks the option "Swap time and date positions",
leaves the settings modal and returns to it, the option will be
unchecked, even though the date and time are, indeed, swapped.
This saves the value of the "Swap time and date positions" option so
that it will match whatever state the user selected last.
Fixes: #165
This doesn't change behavior, but makes the variable `isSwapTime` match
the value of the checkbox state (`cbSwapTimeDate.isChecked()`). Now,
both of them will be either `true` or `false` together, instead of
having opposite values.