Extension Manager improvements

[ChiefOfGxBxL]

• [ ] Should hit the registry on each search
• [ ] Limit search results to 5 for a given query
• [ ] Check for extension updates (compare versions)
• [x] Search results should indicate if a result is already installed
• [ ] onMenuClick menu adder is not implemented when ext is loaded or uninstalled
• [x] Deleting an extension
• [ ] Clicking on install button should trigger loading/indeterminate state. Once the install is complete, the button should change to indicate it is installed, and a card should then be listed on the "Installed" tab. Status (success or errors) should be shown with a Notification (Element)

Reference: #56

[ChiefOfGxBxL]

Search results should indicate if a result is already installed
For this, the search results will simply not display the extension.