logic: Added update checker connecting to online API

* connects to https://www.sourcetrail.com/api/v1/versions/latest
* sends information about OS, platform, license type, current version and a unique user token
* Automatic update check is enabled via Startscreen and performed once every 24 hours on window focus
* Update check can be forced by clicking on "check for new version" on start screen
* renamed TimePoint to TimeStamp

fortune cookie message = In Träumen und im Leben ist nichts unmöglich.

bin/app/data/gui/startscreen/startscreen.css

@@ -42,7 +42,18 @@
 	font-weight: bold;
 }
 
-#updateLabel {
+#updateButton {
+	margin: 1px 0 3px;
+	text-align: left;
+	padding: 0;
+	background: none;
+	border: none;
+	color: #007AC2;
+	font-size: 12px;
+	text-decoration: underline;
+}
+
+#updateCheckbox {
 	color: black;
 	font-size: 12px;
 }

bin/app/user/ApplicationSettings_template.xml

@@ -4,7 +4,8 @@
 <!-- INTEGER: int e.g 123 -->
 <!-- DECIMAL: float e.g 123.456 -->
 <!-- STRING: string e.g hello -->
-<!-- COLOR: int int int int - rgba e.g. 125 125 125 255 -->
+<!-- UUID: string e.g. 00000000-0000-0000-0000-000000000000 -->
+<!-- TIME: string in format "%Y-%m-%d %H:%M:%S" -->
 
 <config>
 	<version><!-- INTEGER: Version number --></version>
@@ -77,6 +78,13 @@
 		</license>
 
 		<accepted_eula_version><!-- INTEGER: last accepted eula version --></accepted_eula_version>
+
+		<token><!-- UUID: identifier for this user --></token>
+
+		<update_check>
+			<automatic><!-- BOOL: whether to check automatically for new software updates --></automatic>
+			<time_stamp><!-- TIME: time of the last update check --></time_stamp>
+		</update_check>
 	</user>
 
 	<network>