chore: speedup channel switching

Correctly clear the chat input

We enable/disable the chat input on various occasions (switching
channels, communities, checking for the person being a contact, etc.).
When we do so, we clear the text of the input which causes all sorts of
internal handlers to get fired, namely the `onTextChanged` part which
parses the text, emojis etc... which is very time consuming. More fixes
here in the followup commits; but by using `clear()` it's much faster,
resets correctly the internal state and also doesn't fire when the input
had been empty already before. Found with QML profiler

ui/imports/shared/status/StatusChatInput.qml

@@ -1342,7 +1342,7 @@ Rectangle {
 
                             onEnabledChanged: {
                                 if (!enabled) {
-                                    text = ""
+                                    clear()
                                     control.hideExtendedArea()
                                 }
                             }