Added maximum size for displaying sent images

additionally:
- made selecting of bmp or gif images possible
- defined a DEFAULT_ICON_SIZE in which most icons are loaded
- displayed attachment button with attachment icon

src/main/resources/fxml/ChatScene.fxml

@@ -101,8 +101,7 @@
 			</GridPane.margin>
 			<buttons>
 				<Button fx:id="attachmentButton" disable="true"
-					mnemonicParsing="false" onAction="#attachmentButtonClicked"
-					text="Add Attachment" />
+					mnemonicParsing="false" onAction="#attachmentButtonClicked" />
 				<Button fx:id="voiceButton" disable="true"
 					onAction="#voiceButtonClicked">
 					<padding>