Effects refactoring minor changes

[daschuer]

Here some minor changes and style improvements, found during testing.

[Be-ing]

/home/runner/work/mixxx/mixxx/src/preferences/dialog/dlgprefeq.cpp: In member function ‘void DlgPrefEQ::slotNumDecksChanged(double)’:
Error: /home/runner/work/mixxx/mixxx/src/preferences/dialog/dlgprefeq.cpp:126:33: error: ‘qEqComboBox’ was not declared in this scope; did you mean ‘pEqComboBox’?
  126 |         gridLayout_3->addWidget(qEqComboBox, deckNo, 1);
      |                                 ^~~~~~~~~~~
      |                                 pEqComboBox
Error: /home/runner/work/mixxx/mixxx/src/preferences/dialog/dlgprefeq.cpp:130:17: error: right operand of comma operator has no effect [-Werror=unused-value]
  130 |                 3,
      |                 ^
Error: /home/runner/work/mixxx/mixxx/src/preferences/dialog/dlgprefeq.cpp:131:17: error: right operand of comma operator has no effect [-Werror=unused-value]
  131 |                 1,
      |                 ^
Error: /home/runner/work/mixxx/mixxx/src/preferences/dialog/dlgprefeq.cpp:132:17: error: right operand of comma operator has no effect [-Werror=unused-value]
  132 |                 1);
      |                 ^
Error: /home/runner/work/mixxx/mixxx/src/preferences/dialog/dlgprefeq.cpp:132:18: error: expected ‘;’ before ‘)’ token
  132 |                 1);
      |                  ^
      |                  ;
Error: /home/runner/work/mixxx/mixxx/src/preferences/dialog/dlgprefeq.cpp:132:19: error: right operand of comma operator has no effect [-Werror=unused-value]
  132 |                 1);
      |                   ^

[daschuer]

Oh sorry, I have messed up one commit when trying to group my random changes reasonable. I have rebased the PR to have all commits building again.

[uklotzde]

LGTM