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.cpp

@@ -664,6 +664,12 @@ void MinecraftProfile::installJarMods(QStringList selectedFiles)
 	m_strategy->installJarMods(selectedFiles);
 }
 
+void MinecraftProfile::installCustomJar(QString selectedFile)
+{
+	m_strategy->installCustomJar(selectedFile);
+}
+
+
 /*
  * TODO: get rid of this. Get rid of all order numbers.
  */