i18n: Add Unicode escape sequence conversion feature

[Sunshine-Planet]

and update Chinese language.

seems like it work for escape sequence and utf8 char in my test.

[mmdanggg2]

Mostly looks good, I'm seeing a couple of NPEs when starting up when set to english though.

java.lang.NullPointerException
	at java.io.Reader.<init>(Reader.java:78)
	at java.io.InputStreamReader.<init>(InputStreamReader.java:113)
	at org.jmc.util.Messages$UTF8Control.newBundle(Messages.java:39)
	at java.util.ResourceBundle.loadBundle(ResourceBundle.java:1518)
	at java.util.ResourceBundle.findBundle(ResourceBundle.java:1482)
	at java.util.ResourceBundle.getBundleImpl(ResourceBundle.java:1370)
	at java.util.ResourceBundle.getBundle(ResourceBundle.java:824)
	at org.jmc.util.Messages.<clinit>(Messages.java:16)
	at org.jmc.gui.Settings.<init>(Settings.java:73)
	at org.jmc.gui.MainWindow.<init>(MainWindow.java:55)
	at org.jmc.Main.runGUI(Main.java:65)
	at org.jmc.Main.main(Main.java:47)

Looks like it's trying to load messages_en.properties which doesn't exist. According to the documentation for newBundle, I think you should specifically check if it exists and return null if not and then just let any IOExceptions be handled further up the stack.