Export Dialog

[pasbi]

The export dialog used to be stored in a static variable, i.e., the same ExportDialog was used throughout the lifetime of the application.
Since this is kind of an anti-pattern (and lead to crashes at application shutdown), the ExportDialog is now created from scratch every time it is called.
As a consequence, it does not remember the last values (e.g., file path, resolution).

• ExportDialog remembers the values within one session
• There is some convenient way how the values are stored (in the scene or as configuration for individual user)
• make sure that, if the file path is stored in the scene, only relative paths are used, as absolute paths presumably become useless on other machines.