Change isometric grid angles type from int to qreal

This MR changes the type of the angles used in the isometric grid from int to qreal, to allow angles such as 26.57, commonly used for example to make isometric art for games.

NOTE: Depending on the decimals that the angle has, the value can be stored in the xml attribute in the document with formats "12.34", "12.3" (if the number is 12.30) or "12" if the number is 12.00). In the case it is stored with decimals, previous versions of krita will load that angle as 0 (default int value). But if the angle is a whole number then it should be no problem.

Formalities Checklist

• I confirmed this builds.
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