Add greeter tab to UI

* adds a tab on startup that shows last connected secnodes
* last nodes saved with qsettings

Change-Id: I2b663a408dc46bd0a0135e723b55d5ef3661bec8
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/30524
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Alexander Zaft <a.zaft@fz-juelich.de>

resources/frappy-gui.qrc

@@ -11,6 +11,7 @@
   </qresource>
   <qresource prefix="icons">
     <file alias="stop">icons/cross-circle.png</file>
+    <file alias="trash">icons/bin.png</file>
     <file alias="plot-add">icons/system-monitor--plus.png</file>
     <file alias="submit">icons/arrow-turn-180.png</file>
     <file alias="connect">icons/plug--plus.png</file>