main: Only set last-shown-version if welcome actually shown

At startup, if the welcome-dialog-last-shown-version GSetting compares
older than WELCOME_DIALOG_LAST_TOUR_CHANGE, Shell attempts to offer the
welcome tour to the user, and then sets that GSetting to the current
version of Shell.

However, showing the welcome dialog can fail. In particular, if
gnome-tour is not installed, WelcomeDialog.open() returns false and the
dialog is not shown; but there are other reasons, such as another modal
dialog already being open. Previously, welcome-dialog-last-shown-version
would nonetheless be updated in this case, so if you subsequently
install gnome-tour (or on the next login, if there is no modal dialog),
the welcome dialog will not be offered until the next time
WELCOME_DIALOG_LAST_TOUR_CHANGE is bumped.

Instead, check whether WelcomeDialog.open() fails; if so, don't update
welcome-dialog-last-shown-version.
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