Comboboxes of DiagramTab won't update when new files are loaded

In DiagramTab comboboxes (to select which data is displayed on X/Y-axis) are not updated when new data files are loaded.

Compare Screenshot Left and Right for one and the same file m200124987.dat

Clearing the session does not help. Steca needs to be closed and restarted

To reproduce this: For the left image: directly load the file after Steca is started For the right image: load a different file (here m248412.dat) and remove (or even clear session) before file m200124987.dat is loaded

Explanation: The content of these boxes are not updated when new files are loaded

Related: Issue #37 (closed)