move text labeling of addExisting vault wizard to string properties file

main/ui/src/main/resources/fxml/addvault_existing.fxml

@@ -15,7 +15,7 @@
 		<Insets top="12" right="12" bottom="12" left="12"/>
 	</padding>
 	<children>
-		<Label text="Open your existing vault by choosing its masterkey file:"/>
+		<Label text="%addvaultwizard.existing.instruction"/>
 		<TextField fx:id="textField" text="path/to/masterkey.cryptomator" disable="true"/>
 		<Button text="file picker" onAction="#chooseFile"/>
 		<Button text="Back" onAction="#goBack"/>

main/ui/src/main/resources/i18n/strings.properties

@@ -1,3 +1,4 @@
+addvaultwizard.existing.instruction=Please choose the masterkey.cryptomator file of your existing vault 
 main.closeBtn.tooltip=Close
 main.settingsBtn.tooltip=Settings
 preferences.autoUpdateCheck=Check for updates automatically

main/ui/src/main/resources/i18n/strings_en.properties

@@ -1,3 +1,4 @@
+addvaultwizard.existing.instruction=Please choose the masterkey.cryptomator file of your existing vault 
 main.closeBtn.tooltip=Close
 main.settingsBtn.tooltip=Settings
 preferences.autoUpdateCheck=Check for updates automatically