Don't update state prematurely in showTranslationFor()

[xiaoyifang]

MainWindow::showTranslationFor() overloads disable the "Pronounce Word"
action, then call ArticleView::showDefinition(). And then immediately
update pronounce availability, Found in Dictionaries list, Back and
Forward buttons. Since ArticleView::showDefinition() loads the requested
page asynchronously, the previous page is still current. Therefore the
"Pronounce Word" action is immediately re-enabled (if the still-current
article has sounds), the other state updates have no effect whatsoever.
Once the new page is loaded, the state is updated again in
MainWindow::pageLoaded() - this time with the desired effect.

So the only effect of the state updates in
MainWindow::showTranslationFor() is to revert the intentional disabling
of the "Pronounce Word" action. Plus waste some CPU time. The
pronunciation-disabling behavior looks better to me and is consistent
with the scan popup's behavior (which immediately hides the
"Pronounce Word" button).