Small usability improvements

[hendrikverdonck]

Gathering of small enhancements which would improve the usability of the GUI

• When renaming the item in the treeview, the existing name should be the start value before renaming (similar to the renaming a file in windows). E.g. currently if you click twice on an item, you can rename it, but it starts with an empty line.
• The tool treeview items should not be editable
• The relative layout sizes of the matplotlib and the tree should be draggable
• If the user cancels a popup, no further windows should appear. E.g. when adding a new dataset the file dialog will still open if the user cancels the data selection popup.