Search bar improvements

New button and key sequence (ESC) to close the search bar.

Clear selection (by not avoiding the search) when the text to search is
deleted, as expected by @pamtbaau at #1220.

src/MainWindow.cpp

@@ -1737,6 +1737,7 @@ unsigned int MainWindow::openSqlTab(bool resetCounter)
     ui->tabSqlAreas->setCurrentIndex(index);
     w->setFindFrameVisibility(ui->actionSqlFind->isChecked());
     w->getEditor()->setFocus();
+    connect(w, SIGNAL(findFrameVisibilityChanged(bool)), ui->actionSqlFind, SLOT(setChecked(bool)));
 
     return index;
 }