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/ftb/FTBProfileStrategy.cpp

@@ -118,6 +118,11 @@ bool FTBProfileStrategy::installJarMods(QStringList filepaths)
 	return false;
 }
 
+bool FTBProfileStrategy::installCustomJar(QString filepath)
+{
+	return false;
+}
+
 bool FTBProfileStrategy::customizePatch(ProfilePatchPtr patch)
 {
 	return false;