Fix a rare bug when `scaleSymbol` is a castable to a Number, and would be...

Fix a rare bug when `scaleSymbol` is a castable to a Number, and would be added to the formatted value, instead of concatenated.

Remove an unneeded temporary variable in `set()`.
Add more details in some JSDoc.
Fix `validate()` so that it throws an error early if `scaleDivisor` is wrongly set to `0`.
Fix `_trimLeadingAndTrailingZeros()` so that it correctly handles the `null` value.
Fix `_onFocusInAndMouseEnter()` so that the `decimalPlacesShownOnFocus` setting is correctly cast to a Number.
Fix `_onFocusOutAndMouseLeave()` so that the `null` value is correctly handle. Also fix the error message shown when the `rawValue` is not stored as a string.
Simplify a ternary condition into a simple `if` one.
Hardcode the `isFocused` state to `false` when calling `_addGroupSeparators()` in `_onFocusOutAndMouseLeave()` so that it does not rely on the order where `this.isFocused` is set back to `false`.

Signed-off-by: Alexandre Bonneau <alexandre.bonneau@linuxfr.eu>