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/ProfileStrategy.h

@@ -23,6 +23,9 @@ public:
 	/// install a list of jar mods into the instance
 	virtual bool installJarMods(QStringList filepaths) = 0;
 
+	/// install a custom jar (replaces the one from the Minecraft component)
+	virtual bool installCustomJar(QString filepath) = 0;
+
 	/// remove any files or records that constitute the version patch
 	virtual bool removePatch(ProfilePatchPtr jarMod) = 0;