map-widget: don't crash if the map QML failed to load

If the QML modules for QtLocation and QtPositioning are
missing the QML in mapwidget.cpp will fail to load,
which can lead to crashes.

To solve the issue check if the QML has loaded and set
a flag 'isReady' to true. If the loading has failed
load another QML which is for showing a red error text
in the lines of `MapWidget.qml failed to load!`.

If the map QML has failed, use a macro in all relevant
MapWidget members to turn them into a NOP. This approach
leaves the rest of the codebase intact - e.g. no checks
in classes which connect to the MapWidget class.

Fixes #596

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>

mobile-widgets/qml/MapWidgetError.qml

@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: GPL-2.0
+import QtQuick 2.0
+
+Item {
+	Text {
+		anchors.fill: parent
+		horizontalAlignment: Text.AlignHCenter
+		verticalAlignment: Text.AlignVCenter
+		color: "red"
+		text: qsTr("MapWidget.qml failed to load!
+The QML modules QtPositioning and QtLocation could be missing!")
+	}
+}