gui: Show a message when no tabs are loaded

[tohojo]

Instead of just showing a blank screen (which can be interpreted as the GUI
hanging), when there are no data files loaded show a message hinting that data
files can be loaded from the menu.

Fixes #293

[richb-hanover]

This is better, but let me propose a couple alternatives to the state after typing flent-gui:

1. Instead of simply displaying "No data files loaded. Open a data file or run a new test in the menu." that window could offer buttons/links to run a test or open a file. Those buttons would have the effect of executing Ctl/Cmd-R for Run new test or Ctl/Cmd-O to open a data file. That would require one click (on either button) to get to where you want to be.

2. When there are no data files open, simply display the "Run new test" window. (Zero clicks to show what newcomers expect.) But also permit that window to respond to the other keyboard shortcuts (Escape, Cmd-Ctl N, O, W, Q, etc.) This would close the "Run new test" window and do whatever's required by that keyboard shortcut.

This lets experienced users who want to open a file (for example) to use the same single keystroke that they would have used before. Thanks for listening.

[tohojo]

Ah yes, good idea with the buttons. Pushed an update, how is this?