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/Proton/RoundedRectangle.qml

@@ -21,7 +21,7 @@ import QtQuick 2.8
 Rectangle {
     id: root
 
-    color: Style.transparent
+    color: "transparent"
 
     property color fillColor        : Style.currentStyle.background_norm
     property color strokeColor      : Style.currentStyle.background_strong