Add (Double)SliderSpinBox to Python API

The first commit fixes a bug where Kis(Double)SliderSpinBox's fastSliderStep was always int, which affected the Select Shape Tool's Opacity slider.

The second wraps KisSliderSpinBox and KisDoubleSliderSpinBox so they can be used from Python.

The third adds the Int/Double ParseSpinBox to Python and has the SliderSpinBoxes inherit from it.

Test Plan

0. Before recompiling, delete [buildfolder]/plugins/extensions/pykrita/sip/_tmp so that the Python bindings will be regenerated.
1. Create a shape and select it. In the Select Shape Tool's Tool Options, hold Ctrl and drag the Opacity slider to test that fastStep is correctly .1.
2. Use this script: https://invent.kde.org/-/snippets/2894 in the Scripter to test SliderSpinBox and DoubleSliderSpinBox's functions, as shown in the image.
3. Use this script: https://invent.kde.org/-/snippets/2909 in the Scripter to test IntParseSpinBox and DoubleParseSpinBox's functions, as shown in the image.

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Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.

Reminder: the reviewer is responsible for merging the patch, this is to ensure at the least two people can build the patch. In case a patch breaks the build, both the author and the reviewer should be contacted to fix the build. If this is not possible, the commits shall be reverted, and a notification with the reasoning and any relevant logs shall be sent to the mailing list, kimageshop@kde.org.