Create “Welcome screen” that replaces window where nearly all widgets are in disabled state

This merge request replaces the window contents when the Okular application is started without giving it archive to open. The current user interface has all its widgets in disabled state and the only way to open document is to use menu bar (not shown in pictures) or keyboard shortcuts. I think that it is hard to figure out what to do for unexperienced users.

The new user interface adds two buttons directly to the center of window: one for opening document and the second for opening the Recents menu. When document is opened, this “screen” is replaced by the normal user interface of Okular. When the last document is closed, the welcome screen does not reopen.

This changes only Okular's main window, not Okular KPart, so it will not break any existing usages of the KPart.

Very similar screen is in Ark since utilities/ark!47 (merged).

Before

After change