feat: hide pre-releases by default

[skaldarnar]

This PR adds a new checkbox to the launcher settings view for the user to enable pre-releases. This checkbox is deselected by default. The launcher will only show pre-releases (including release candidates) when the user explicitly opts in to show them via the settings menu.

• when the user selects a pre-release and then unchecks the property the game selection box is empty
• all the pre-releases are still fetched on start-up (as before)
• I'm not sure the property exposed by LauncherSettings is always correctly initialized

Somewhat addresses #479

[skaldarnar]

It seems to pick up the setting as saved on last run.

• start the launcher with option disabled ⇒ no pre-releases
• enable the option via settings menu ⇒ pre-releases are shown
• close the launcher
• re-start the launcher ⇒ option is still enabled, pre-releases are shown

Also, switching the option on and off seems to work fine. If a pre-release version was selected when the option is disabled the box will stay empty. Even though the download button is enabled in that case, clicking it does not do anything...

[skaldarnar]

Properly fixing the situation that leaves the game release checkbox would require a bit (if not "a lot") more work.

We currently update the selection whenever we change the selected game profile, but this part is not properly hooked up with the rest of the properties.

https://github.com/MovingBlocks/TerasologyLauncher/blob/3a1bc1cbc004bb33ffb44e549c7418bb1def5c03/src/main/java/org/terasology/launcher/ui/ApplicationController.java#L177-L192

I'm not really sure how to best resolve this, and would prefer to clean up other parts of the code beforehand. For instance, we could improve a lot on LauncherSettings and BaseLauncherSettings by providing more Propertys there.