GH-1314 add UI for custom minecraft jar addition

Also changes the text of the jar mod addition button.
It should be clearer what it does and hopefully will not confuse
as many people.

api/logic/minecraft/MinecraftProfile.h

@@ -58,6 +58,9 @@ public:
 	/// install more jar mods
 	void installJarMods(QStringList selectedFiles);
 
+	/// install more jar mods
+	void installCustomJar(QString selectedFile);
+
 	/// DEPRECATED, remove ASAP
 	int getFreeOrderNumber();