Refresh Button for cached data from DS Service

[AlexanderWert]

create a button to refresh the extensions-cache for extensions provided by DynamicSpotterService.

This is needed for example when dynamicSpotter server is restarted with a new set of extensions

[DenisKnoepfle]

Currently if it is needed for testing purposes this can be achieved by

• shutting down the DS service
• closing an editor where the data has already been cached
• reopen it and trying to access the extensions via the Add button

It will say there is no connection and clear all cached data. After DS service is restarted it will fetch the new data.

But sure, a button to do this would be more user-friendly and will be implemented soon.

[DenisKnoepfle]

Before implementing this feature some questions concerning what the button will do should be clarified first:

• Only clear cache?
• What should happen with opened editors/views?
• Should they be closed or tried to be refreshed (by rebuilding the editor input from file or from the current model inside the editor; if from file: what happens with unsaved data)?
• What should happen if new extensions fetched from DS Service no longer match some configured extensions, e.g. due to a changed name or config parameters that were modified or no longer exist?
• Should non-matching parameters just be ignored or should the whole editor input be declared invalid?

Until theses issues are clear I would suggest to implement the button in the context menu giving it only the functionality to "Clear Cache" (but loading outdated extensions from file will fail then until the complete validation and recovery of the input is implemented; currently it only checks whether the file can be parsed but not if occurring extensions have their parameters configured correctly).

[DenisKnoepfle]

Enhancement is included in #60.

[DenisKnoepfle]

Updated info message in #72, but ExtensionItem needs refactoring first in order to fully clear all cached data. This will be addressed after #51, #68 and #69.

[DenisKnoepfle]

Removed caching inside extension items in #95.