DlgPrefEQ: fix loading/saving main EQ parameters

[ronso0]

https://bugs.launchpad.net/mixxx/+bug/1983764
https://bugs.launchpad.net/mixxx/+bug/1983789 likely a regression from effects refactoring

Verify lp:1983764 is fixed:

• load 'Graphic EQ'
• click 'Reset Parameter' (don't touch sliders)
• 'frequency image' and output level should not change
• restart Mixxx
• 'frequency image' and output level should still be unaffected

Verify lp:1983789 is fixed:

• load 'Parametric EQ'
• click 'Reset Parameter'
• load 'Graphic EQ' (all sliders are centered, don't touch!)
• close & restart Mixxx
• all sliders should still be still be centered

Unfortunately, that pref page is still a mess, most (all?) selections are immediately applied (okay) and stored to config (not okay since is clicking Cancel does not reset options to prevous state).
Eventually I'll port #4667 to main one day.

[ronso0]

Remaining issues:

• scrolling over the main EQ combobox switches the effect, effectively clearing the previous configuration
• scrolling over main EQ parameter sliders moves the slider but doesn't change the effect parameter

Preferrable fix:
ignore all scroll events sent to sliders & comboboxes, exactly as in Sound Hardware page

[ronso0]

Preferrable fix: ignore all scroll events sent to sliders & comboboxes, exactly as in Sound Hardware page

Done, ready for review.

[ronso0]

For some reason the division of int / int did not return a double but int.
Casting dividend & divisor to double fixes that 🤷

[daschuer]

Yes, the compiler keep the type. Once one of the values is double, the result is also double.
Just use static_cast<double>() here to be more in line with the other code.

[ronso0]

Okay, done.

[daschuer]

@ferranpujolcamins Thank you for review. My comments are also solved.
Thank you @ronso0 for noticing and fixing the issue.
I think we need urgently branch out a Beta that we receive more Test.