desktop: store pointer to parent in tab-widgets

Make it possible for the individual tab-widgets to access the
parent widget. In principle this could have been done by
downcasting the pointer returned by parent(), but this makes
it explicit.

The goal here is to store information on the selection,
current dive, etc. without repeating it in every subwidget.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

desktop-widgets/tab-widgets/TabBase.cpp

@@ -1,6 +1,10 @@
 // SPDX-License-Identifier: GPL-2.0
 #include "TabBase.h"
 
+TabBase::TabBase(MainTab *parent) : parent(*parent)
+{
+}
+
 void TabBase::updateUi(QString)
 {
 }