Split up and rename paddle_menu_manager_ui.test

The user interface tests have been split up over multiple files. The tests concerning menu creation dialogs are placed in paddle_menu_manager_menu_form.test. This means that the name of paddle_menu_manager_ui.test is no longer correct, this does not contain all UI tests anymore.

I suggest to split these tests up in the following files:

1. paddle_menu_manager_overviews.test: contains listings and overviews, these all need to have some pre-generated menus.
   • testLoadLanguage() -> this function is not correct, this actually tests the menus block.
   • testMenuOverview()
   • testMenuOverviewTypes()
2. paddle_menu_manager_pages.test: contains simple page callbacks that do not require any pre-generated content.
   • testPlaceholderLink()

The documentation and classnames can be changed accordingly.