Begin yet another model refactor

Use native ListModel which require a lot of changes, but should be
much faster than the old way which exponentially slowed down to a crawl.
Also fix some popup bugs (leave/forget).

Not working yet: side pane keyboard controls, proper highlight,
room & member filtering, local echo replacement

src/gui/PythonBridge/EventHandlers.qml

@@ -1,87 +0,0 @@
-// SPDX-License-Identifier: LGPL-3.0-or-later
-
-import QtQuick 2.12
-
-QtObject {
-    function onExitRequested(exitCode) {
-        Qt.exit(exitCode)
-    }
-
-
-    function onAlertRequested() {
-        if (Qt.application.state !== Qt.ApplicationActive) {
-            window.alert(window.settings.alertOnMessageForMsec)
-        }
-    }
-
-
-    function onCoroutineDone(uuid, result, error, traceback) {
-        let onSuccess = py.privates.pendingCoroutines[uuid].onSuccess
-        let onError   = py.privates.pendingCoroutines[uuid].onError
-
-        delete py.privates.pendingCoroutines[uuid]
-
-        if (error) {
-            const type = py.getattr(py.getattr(error, "__class__"), "__name__")
-            const args = py.getattr(error, "args")
-
-            if (type === "CancelledError") {
-                console.warn(`python: cancelled: ${uuid}`)
-                return
-            }
-
-            if (onError) {
-                onError(type, args, error, traceback)
-                return
-            }
-
-            console.error(`python: ${uuid}\n${traceback}`)
-
-            if (window.hideErrorTypes.has(type)) {
-                console.warn(
-                    "Not showing error popup for this type due to user choice"
-                )
-                return
-            }
-
-            utils.makePopup(
-                "Popups/UnexpectedErrorPopup.qml",
-                window,
-                { errorType: type, traceback },
-            )
-        }
-
-        if (onSuccess) onSuccess(result)
-    }
-
-
-    function onLoopException(message, error, traceback) {
-        // No need to log these here, the asyncio exception handler does it
-        const type = py.getattr(py.getattr(error, "__class__"), "__name__")
-
-        if (window.hideErrorTypes.has(type)) {
-            console.warn(
-                "Not showing error popup for this type due to user choice"
-            )
-            return
-        }
-
-        utils.makePopup(
-            "Popups/UnexpectedErrorPopup.qml",
-            window,
-            { errorType: type, message, traceback },
-        )
-    }
-
-
-    function onModelUpdated(syncId, data, serializedSyncId) {
-        if (serializedSyncId === "Account" || serializedSyncId[0] === "Room") {
-            py.callCoro("get_flat_mainpane_data", [], data => {
-                window.mainPaneModelSource = data
-            })
-        }
-
-        window.modelSources[serializedSyncId] = data
-        window.modelSourcesChanged()
-    }
-}