Desktop UI: allow user defined cylinder as default

Currently, it was only possible to use a hard-coded default
cylinder in the preferences. At the same time, the user is
allowed to add own cylinders to a dive (by just typing in
a new name and cylinder properties). These own cylinders
could not be selected in the preferences, requiring the
user to manually add this every dive.

Not sure the reason for all this was intentional, or just
an overlooked aspect in the implementation. It appeared
that the selection list in the UI was constructed before
any logbook was parsed, so at that moment, there are only
hard-coded cylinders.

The fix is simple. Refresh the UI of the preferences just
before it is shown. While opening the logbook, a new
list of cylinders, including the own cylinders is
constructed, so the UI is refreshed to that.

While a simple change, there is a use case that might be
considered strange. Open a logbook, choose new logbook,
and change the default cylinder preference to an own
(from the previously opened logbook). Do not add a dive
with the new (own) cylinder and save this logbook.
Reopen this new logbook, and see that the default
cylinder in the preferences is empty. This is logical,
as the list of own possible cylinders is constructed
from the (new) logbook, that has no dive with that
specific own cylinder. I consider this acceptable
behavior, as it can be also be used to copy own
cylinders to a new logbook.

Fixes: #821

Proposed-by: Davide DB <dbdavide@gmail.com>
Signed-off-by: Jan Mulder <jlmulder@xs4all.nl>

desktop-widgets/mainwindow.cpp

@@ -774,6 +774,11 @@ void MainWindow::showProfile()
 
 void MainWindow::on_actionPreferences_triggered()
 {
+	// the refreshPages() is currently done for just one
+	// reason. Allow the user to define a default cylinder that
+	// is not hardcoded but coming from the logbook.
+	PreferencesDialog::instance()->refreshPages();
+
 	PreferencesDialog::instance()->show();
 	PreferencesDialog::instance()->raise();
 }