Syncing the application name with the name of the desktop file

Gwenview application name stayed 'gwenview' while the .desktop file switched to the org.kde.*something* naming scheme.

While currently not that important - in the recent future, syncing these two will be needed for the application to properly work with the things like:
- recent/favourite/high scored documents for gwenview in the task-bar
- remembering the prefered application for specific files
- probably something else that I can not remember atm :)

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