Fixed navigating articles with keyboard and shortcuts for switching windows.

* articleview.cc: fixed activating link with \<Enter\>. Before my changes it was impossible to activate current link in article window when navigating with \<Tab\>, \<Shift+Tab\>.
* fulltextsearch.ui: fixed FullTextSearchDialog tab order. Before my changes tab order in this dialog was completely messed up. Now it has the following logic: first searchLine, then headwordsView (can be  navigated with arrow keys), then other UI elements from top to bottom.
* mainwindow.hh, mainwindow.cc: fixed switching between UI elements via shortcuts. Apart from ensuring more consistent behavior of existing shortcuts and moving duplicated code in MainWindow::addGlobalAction method, added shortcuts to focus article view and full text search dialog.