Compatibility information for extensions in Joomla 2.5

### Issue
There is no simple way for a user to see if an installed extension will also be available after the upgrade to Joomla! 3.3.

### Proposal
This PR will collect the compatibility information from the updateserver stream. It will show them as a tooltip in the Extension Manager

### Technical Details
I admit it's a bit a hack, but I had some fun digging through our updater and learned quite a bit :smile: 
What this does is:
* During searching for available updates, this PR will store the information from the `targetplatform` attribute in the updateserver stream into an array.
* At the end of the update searching process, it will take the values for the currently installed version and for the latest available version and stores them into the currently not used `system_data` column in the `#__extensions` table. It does this as a JSON string. This will create an additional query for extension which have an updateserver.
* In the Extension Manager -> Manager view, it will now show a tooltip icon behind the version if compatibility information is available. The tooltip will show the information for the current and the latest available version

### Testing
Test with 3rd party extensions. Those using an updateserver should show the information after you have searched for available updates (doesn't matter if there are updates or not).

#### Feedback welcome