Importing Thematic Grids deletes all existing grids

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What steps will reproduce the problem?
1. You have more grids than the grids from the downloadable XML file or have 
changed that grids
2. Import the grids from the XML file again

What is the expected output? What do you see instead?
The imported grids should be added to the other grids if they are new (names 
does not exists). If the same name exists the user should be asked if he wants 
to overwrite same named grids or not. There should come up a MessageBox with 
Yes-No-Cancel buttons once per import, if same grids are detected.
The current import behavior is that all existing grids are deleted and the 
grids from the XML file are added.

Original issue reported on code.google.com by dirk.mei...@gmail.com on 19 May 2011 at 7:38

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Original comment by dirk.mei...@gmail.com on 26 May 2011 at 6:20

• Changed state: Started
• Added labels: Priority-High

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Original comment by krauseda...@web.de on 1 Jun 2011 at 7:01

• Added labels: Milestone_1

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The user is now asked if he wants to overwrite existing grids with same names 
or not. If he don't want to overwrite them, only grids with "new" names are 
imported.

Original comment by dirk.mei...@gmail.com on 8 Jun 2011 at 9:25

• Changed state: Fixed