(needs collaboration) Allow multiple tabs in embedded terminal emulator

In the Kate text editor, the feature of adding new tabs in its embedded terminal emulator (the user can press there Ctrl+Shift+T several times) has been added. Krusader can have that, too. In this MR there's source code that allows adding new tabs in the embedded terminal of Krusader (the user can press there Ctrl+Shift+T several times), although collaboration from other developers is needed because, in the current state of the source code, e.g. a tab bar is not seen ─so the user can not switch back and forth between the tabs of the terminal.