Replace the pngs of the step buttons with the svgs from the previous branch.

Adjust the code so they are rendered with the maximum available height.

Ensure minimal height of 24 pixels like the pngs. Take border offsets into account.

Fix the method `mousePressEvent` so that it also works with the larger step buttons. The main change is the adjusted check for the y coordinate.

All other changes are just refactoring and reformatting of the method's code.

…teps

Extract the computation of the selected step into a dedicated method in `MidiClipView`. The removes duplicated code in the mouse press and wheel event handling code.

The method `getStep` returns an `std::optional` which contains the value of the step if the calculation was successful. It also contains the check for the height.

The calculation with floats was removed in `getStep` as it should not be necessary.

Adjust `MidiClipView::wheelEvent` so that it now computes the step in the same way as the mouse event handler. This reinstates the functionality to set the note velocities with the mouse wheel even if the beat pattern is enlarged.

Adjust `data/themes/CMakeLists.txt` so that SVGs in the theme directories are also installed.

Fix artifacts that occur when Qt is requested to render SVGs into very small pixmaps. This is done by making sure that the SVG is always rendered into pixmaps that are at least 24x24 in size (the size of the original PNG pixmap).

Adjust the pixmap drawing code so that a potentially larger source pixmap is drawn into a smaller target pixmap.