Minor improvements to Export GUI

The following are some small issues that should be fixed in the "export gui".

Steps to open export gui:

1. start ert gui with poly_example and run ensemble experiment
2. Select Manage experiments > expand "single test run" > select ensemble
3. Select Parameters tab. Click export button on the bottom of the pane

Issues

1. When resizing dialog the center text area should be the one resized not the top and bottom elements (or make the whole dialog fixed size) (1)
2. The file name shown should have its whole path printed in the text area (2)
3. The text area should either be cleared between presses of export or the focus should be on the latest entry in the text area. Now the focus is always on top