Resolves: #9356

In the "Palettes" sidebar, some icons are zoomed in or out to make them more legible. (For example, clefs are shown at 0.8x magnification; fingering indicators are shown at 1.5x magnification.)

Previously, items in custom palettes were not being scaled. As a result, clefs in custom palettes looked very large, and fingering indicators in custom palettes looked very small.

This PR fixes that behavior; now, when an item is added to any palette, it is given a magnification value according to what type of element it is.

Caveats:

• You may need to click the "reset palette" button in order for these changes to take effect.

• Some elements—in particular, action icons—still appear slightly too large in custom palettes, for reasons I don't entirely understand. [Fixed]

• Some palettes have cells with varying magnifications. Previously, the logic to accomplish this was spread between palettecreator.cpp and palette.cpp. I've consolidated this logic in palette.cpp in the scaleCell() method.

• Previously, some tempo text icons were displayed at 0.975x scale, and others were displayed at 0.845x scale. To keep the logic in scaleCell() simple, I split the difference and set them both to 0.9x scale.

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• I created the test (mtest, vtest, script test) to verify the changes I made