Always enable 'Check for Updates' menu button

[LoayGhreeb]

• "Help -> Check for Updates" button in the menu bar will be enabled regardless of whether the "auto-check for updates at startup" preference is enabled or disabled.
• Fixed an issue where changing the "Preferences -> Network -> Check for latest version online" value was not being saved.

Closes https://github.com/JabRef/jabref-issue-melting-pot/issues/470

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.

[HoussemNasri]

LGTM 👍

[calixtus]

Does this change mean, jabref will now always check on startup, if a new version is present?
This can be very annoying, if you explicitly want to stay on an older version, if the newer one has a bug.

[LoayGhreeb]

This method will only execute when using Help -> Check for updates, so it shouldn't perform any checks.

The other method, checkForNewVersionDelayed() (Line 70), used to check for updates on startup, will only be called if it is enabled in the preferences.

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

Lines 111 to 119 in 7e4e7d3

	EasyBind.subscribe(preferencesService.getInternalPreferences().versionCheckEnabledProperty(), enabled -> {
	if (enabled) {
	new VersionWorker(buildInfo.version,
	dialogService,
	taskExecutor,
	preferencesService)
	.checkForNewVersionDelayed();
	}
	});