GODT-22: Frontend-backend

- GODT-1246 Implement settings view.
- GODT-1257 GODT-1246: Account and Help view
- GODT-1298: Minimal working build (panics)
- GODT-1298: loading QML (needs Popup window)
- GODT-1298: WARN: Adding PopupWindow not possible!
    In therecipe qt the `quickwidgets` classes are within `quick` module, but
    forgot to add library and include paths into cgo flags. Therefore
    compilation fails and it would be hard to patch therecipe in order to
    fix it.

    I am not sure if rewrite PopupWindow into go would make any difference,
    therefore I decided to use normal QML Window without borders.
- GODT-1298: Rework status window, add backend props, slots and signals.
- GODT-1298: Users
- GODT-1298: Login
- GODT-1298: WIP Help and bug report
- GODT-1178: MacOS dock icon control
- GODT-1298: Help, bug report, update and events
- GODT-1298: Apple Mail config and Settings (without cache on disk)

internal/frontend/qml/SettingsView.qml

@@ -0,0 +1,71 @@
+// Copyright (c) 2021 Proton Technologies AG
+//
+// This file is part of ProtonMail Bridge.
+//
+// ProtonMail Bridge is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// ProtonMail Bridge is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with ProtonMail Bridge.  If not, see <https://www.gnu.org/licenses/>.
+
+import QtQuick 2.13
+import QtQuick.Layouts 1.12
+import QtQuick.Controls 2.13
+import QtQuick.Controls.impl 2.13
+
+import Proton 4.0
+
+ScrollView {
+    id: root
+
+    property var colorScheme
+    property var backend
+    default property alias items: content.children
+
+    signal back()
+
+    property int _leftRightMargins: 64
+    property int _topBottomMargins: 68
+    property int _spacing: 22
+
+    clip: true
+    contentWidth: pane.width
+    contentHeight: pane.height
+
+    RowLayout{
+        id: pane
+        width: root.width
+
+        ColumnLayout {
+            id: content
+            spacing: root._spacing
+            Layout.maximumWidth: root.width - 2*root._leftRightMargins
+            Layout.fillWidth: true
+            Layout.topMargin: root._topBottomMargins
+            Layout.bottomMargin: root._topBottomMargins
+            Layout.leftMargin: root._leftRightMargins
+            Layout.rightMargin: root._leftRightMargins
+        }
+    }
+
+    Button {
+        anchors {
+            top: parent.top
+            left: parent.left
+            topMargin: 10
+            leftMargin: 18
+        }
+        colorScheme: root.colorScheme
+        onClicked: root.back()
+        icon.source: "icons/ic-arrow-left.svg"
+        secondary: true
+        horizontalPadding: 8
+    }
+}