Link preview messageeventmodel parameters

This move the finding of links and the creation of a `linkpreviewer` into c++.

- The links are now extracted from the text in `texthandler`
- The `messageeventmodel` now creates and stores `linkpreviewers` for events that have links in the current room.

Two new model roles have been created to let a text delegate know when the link preview should be shown (`showLinkPreview`) and pass the link previewer (`linkPreviewer`). Empty link previewer are returned where link don't exist so the qml doesn't have to have checks for whether the parameters are undefined.

src/qml/Component/Timeline/MessageDelegate.qml

@@ -35,7 +35,8 @@ TimelineContainer {
         }
         LinkPreviewDelegate {
             Layout.fillWidth: true
-            room: currentRoom
+            active: !currentRoom.usesEncryption && currentRoom.urlPreviewEnabled && Config.showLinkPreview && model.showLinkPreview
+            linkPreviewer: model.linkPreview
             indicatorEnabled: messageDelegate.isVisibleInTimeline()
         }
     }