* Fix for Icons and comboboxes mismatch in arpeggiator in Instrument Editor #5494
(https://github.com/LMMS/lmms/issues/5494)
Introduce a static const int variable for the default height of a ComboBox.
Set this height already in the constructor of the ComboBox object.
Update all modules setting the height of a ComboBox object to make use of the new constant.
* Replace 'const int' by 'constexpr int' after review.
Removes some repeated calls to Qt's font layouting by using QStaticText
in FxLine and removing the overridden method ComboBox::sizeHint.
Unifies Mixer::peakValueLeft and Mixer::peakValueRight into
Mixer::getPeakValues so the array is only iterated once.