Fixed various bugs

These are:
* different size of addContact- and SettingsButton
* default icons in light mode for users and groups (even though they are
currently just the version used in dark mode)
* wrong preferred size of unnamed "Login" label in LoginScene
* unopenable LoginScene for some OS (Debian)
* white screen when the current scene is switched

Additionally cleaned up code a bit in MessageControl and
LoginScene(.java)

client/src/main/resources/fxml/ChatScene.fxml

@@ -66,7 +66,7 @@
 				<Region id="transparentBackground" prefHeight="77.0" prefWidth="115.0" />
 				<VBox id="transparentBackground" alignment="CENTER_RIGHT" prefHeight="200.0" prefWidth="100.0" spacing="5.0">
 					<children>
-						<Button fx:id="settingsButton" mnemonicParsing="true" onAction="#settingsButtonClicked" text="">
+						<Button fx:id="settingsButton" mnemonicParsing="true" onAction="#settingsButtonClicked" prefHeight="30.0" prefWidth="30.0" text="">
 							<padding>
 								<Insets bottom="5.0" left="5.0" right="5.0" top="5.0" />
 							</padding>
@@ -74,7 +74,7 @@
 								<Insets />
 							</VBox.margin>
 						</Button>
-						<Button mnemonicParsing="true" onAction="#addContactButtonClicked" text=" + ">
+						<Button mnemonicParsing="true" onAction="#addContactButtonClicked" prefHeight="30.0" prefWidth="30.0" text=" + ">
 							<padding>
 								<Insets bottom="5.0" left="5.0" right="5.0" top="5.0" />
 							</padding>

client/src/main/resources/fxml/LoginScene.fxml

@@ -32,7 +32,9 @@
             <Insets bottom="5.0" left="5.0" right="5.0" top="5.0" />
          </padding>
       </Label>
-		<Label alignment="TOP_CENTER" contentDisplay="CENTER" prefHeight="33.0" prefWidth="87.0" text="LOGIN" textAlignment="CENTER">
+		<Label alignment="TOP_CENTER" contentDisplay="CENTER"
+			prefHeight="33.0" prefWidth="110.0" text="LOGIN"
+			textAlignment="CENTER">
 			<font>
 				<Font size="26.0" />
 			</font>