add settings directory link to Help menu

[ronso0]

A menu item is a real shortcut compared to clicking the button in Pref > Library and or pasting the path into a file manager bar.

Closes #11667

[Swiftb0y]

LGTM

[ronso0]

Whooop, that was a quick fix : )

[m0dB]

Unfortunately this PR has broken opening the Preferences dialog on macOS.

Selecting the menu item, or using the shortkey, has no effect. When I comment out the code introduced by this PR, it works as expected.

It looks like the &WMainMenuBar::showPreferences signal is never emitted. I connected &WMainMenuBar::showAbout to &MixxxMainWindow::slotOptionsPreferences to check, and with that the About menu item triggers the preferences dialog just fine.

[m0dB]

I found the cause: the use of &Settings here:

QString settingsDirTitle = tr("&Settings directory");      

It looks like this interferes with the Preferences menu on macOS (I guess due to the transition from Preferences to Settings). If I change this to

QString settingsDirTitle = tr("&User settings directory"); 

the preference menu starts working again. So it looks like with this PR the Preferences menu triggers this pHelpSettingsDir action instead (which BTW, doesn't seem to do anything on macOS)

@ronso0 @Swiftb0y how to proceed? Will you take care of this or do you want me to create a PR?

[m0dB]

@ronso0 @Swiftb0y This PR fixes both issues. This PR should IMO be merged ASAP as having no preferences menu on macOS is pretty bad.

#11679