wxSpinCtrlDbl SetIncrement() and SetDigits() interaction

[steve_lamerton]

Under GTK GetIncrement and SetIncrement have a few bugs:

1. SetIncrement clears the current page size if one has been set.
2. GetIncrement actually returns the page size and not the increment size.
3. Calling SetIncrement makes no difference as by default the spin control is created with a precision of 1 and so (for example) calling SetIncrement(0.1) and then SetValue(1.1) with just display 1. This can be worked around by the user calling SetDigits before but should probably be automated.

Attached is a patch for issues 1 & 2.

[vadz]

I'll apply your patches for the first two points, thanks!

Concerning the remaining one, I'm not sure how does this work in the generic version. If it doesn't do anything fancy (and I don't think it does), we should probably just document that you may want to call SetDigits() in SetIncrement() documentation. Of course, ideally we'd implement the smart behaviour for both versions but I'm not sure if it's worth the trouble.

[VZ]

(In [65671]) Fix wrong return value of wxGTK wxSpinCtrlDouble::GetIncrement().

Due to confusion in gtk_spin_button_get_increments() parameters order the page
size was returned instead of the increment.

See #12342.

[VZ]

(In [65672]) Preserve page size when changing increment in wxGTK wxSpinCtrlDouble.

The page size was wrongly reset when changing the increment before.

See #12342.

[SJL]

(In [72514]) Update the wxSpinCtrlDouble documentation so SetIncrement refers to SetDigits

Closes #12342