Convert Manage keywords dialog to JavaFX (#4701)

* Convert Manage keywords dialog to JavaFX

* Improve code

Author: tobiasdiez

src/main/java/org/jabref/gui/JabRefFrame.java

@@ -67,7 +67,6 @@
 import org.jabref.gui.actions.ManageCustomExportsAction;
 import org.jabref.gui.actions.ManageCustomImportsAction;
 import org.jabref.gui.actions.ManageJournalsAction;
-import org.jabref.gui.actions.ManageKeywordsAction;
 import org.jabref.gui.actions.NewDatabaseAction;
 import org.jabref.gui.actions.NewEntryAction;
 import org.jabref.gui.actions.NewEntryFromPlainTextAction;
@@ -81,6 +80,7 @@
 import org.jabref.gui.actions.SimpleCommand;
 import org.jabref.gui.actions.StandardActions;
 import org.jabref.gui.dialogs.AutosaveUIManager;
+import org.jabref.gui.edit.ManageKeywordsAction;
 import org.jabref.gui.edit.MassSetFieldsAction;
 import org.jabref.gui.exporter.ExportCommand;
 import org.jabref.gui.exporter.ExportToClipboardAction;