Improve behaviour for empty modpacks and when no mods at all are installed:

Only show enable all / disable all buttons for all add-ons when at
least one add-on is installed. When no add-on ist installed, don't
show any buttons or checkboxes.

Added is_modpack flag to ModSpec to distinguish empty modpacks from
normal mods and check this flag in mod selection gui so that empty
modpacks are not treated like mods that can be enabled or disabled.

src/mods.h

@@ -53,6 +53,8 @@ struct ModSpec
 	//if normal mod:
 	std::set<std::string> depends;
 	std::set<std::string> unsatisfied_depends;
+
+	bool is_modpack;
 	// if modpack:
 	std::map<std::string,ModSpec> modpack_content;
 	ModSpec(const std::string name_="", const std::string path_="",
@@ -61,6 +63,7 @@ struct ModSpec
 		path(path_),
 		depends(depends_),
 		unsatisfied_depends(depends_),
+		is_modpack(false),	
 		modpack_content()	
 	{}
 };