Qt CoAP: Add alt texts

qdoc: 'QtLabsPlatform': \image qtlabsplatform-colordialog-gtk.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtLabsPlatform': \image qtlabsplatform-filedialog-gtk.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtLabsPlatform': \image qtlabsplatform-folderdialog-gtk.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtLabsPlatform': \image qtlabsplatform-fontdialog-gtk.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtLabsPlatform': \image qtlabsplatform-menu.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtLabsPlatform': \image qtlabsplatform-menubar.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtLabsPlatform': \image qtlabsplatform-messagedialog-android.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtLabsPlatform': \image qtlabsplatform-messagedialog-informative-android.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtLabsPlatform': \image qtlabsplatform-systemtrayicon.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtLabsPlatform': \image qtlabsplatform-systemtrayicon-menu.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtLabsPlatform': \image qtlabsplatform-systemtrayicon-message.png is without a textual description, QDoc will not generate an alt text for the image.