revamp kdeconnect-handler to support full control by GUI

Current implementation of kdeconnect-handler did not support controlling the function of kdeconnect-handler completely through GUI. This patch enables that by adding knobs to the GUI and doing necessary changes to the cpp.

This is particularly useful for the Windows Store release, since we can only provide one entry point and currently, the handler.exe is not exposed anywhere. I plan to expose it through the system tray as another entry along with Send File, SMS Messages and others.