fix bug when switching between open dialogs

[beanbeanjuice]

When switching between open dialogs, it switches only the dialog title rather than the actual content of the dialog.

To add further to this, warn the user that the current dialog is about to be replaced (with any data entered lost) and ask them if they want to continue or return to the previous dialog.

Example:

1. Select Edit -> Adjust geometric parameters
2. Select File -> Load example