Add keyboard shortcut Ctrl + , to open preferences

[jeanprbt]

This PR adds the keyboard shortcut Ctrl + , to open the preferences.

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[jeanprbt]

I am just struggling adding this new keyboard shortcut to the table of shortcuts in the Key bindings preferences tab. Any idea ? Every shortcut seems to be added to the keyBindingsRepository of the KeyBindingsTabView class, but I do not find where it is initalized.

[Siedlerchr]

According to your other PR this must be now keyboard shortcut!

[Siedlerchr]

I am just struggling adding this new keyboard shortcut to the table of shortcuts in the Key bindings preferences tab. Any idea ? Every shortcut seems to be added to the keyBindingsRepository of the KeyBindingsTabView class, but I do not find where it is initalized.

It's handled over the StandardActions enum

jabref/src/main/java/org/jabref/gui/MainMenu.java

Line 157 in 2ba3b2a

factory.createMenuItem(StandardActions.SHOW_PREFS, new ShowPreferencesAction(frame, dialogService)),

From there on it goes over to the createMenuItem stuff where it is associated with the menuItem