Second attempt to fix shortcuts and focus issues.

Changes since https://github.com/goldendict/goldendict/pull/479:
* Removed ```focusFullTextSearchAction```;
* moved setting group under ```if``` statement in ```MainWindow::showFullTextSearchDialog()```;
* set focus to results list in ```FullTextSearchDialog``` after first result is found;
* fixed ```FullTextSearchDialog``` and ```DictHeadwords``` overlapping.

Note: overlapping issue with detached dock widgets (for example, detached search pane) still exists: widget is not brought to front when it receives focus. My experiments indicate that the only way to move widget A over sibling widget B is calling ```lower()``` on B. The naive approach would be calling ```lower()``` on all existing detached children of main window. The problem with this simple approach is that relative other widgets' z-order would be changed.
By the way, just found another bug: detached search pane is not always hidden with main window. Maybe all these visibility issues are desktop environment-dependant (I am using XFCE).