Should be unnecessary since we are now checking if `cTabActUav` exists.
Unset `cTabUAVcams` / `cTabHcams` when closing  UAV / Hcam GUI.
Altered F1-F4 shortcuts and tablet on-screen buttons to use restructured GUI load and unload (mainly to register event handlers). Restructured UAV, Hcam and MSG GUIs that I missed on the first pass. Also removed unnecessary call of `main\onload.sqf` (already gets called in the `onLoad` event of the tablet main GUI)
Altered tablet desktop shortcuts to use restructured GUI load and unload (mainly to register event handlers).
Check that `cTabActUav` / `cTabActHcam` is defined before switching to full screen UAV / Hcam view.
Define `IDC_CTAB_ACTMSGTXT` and use it where `actMSGtxt` has been referenced.
Added missing MSG icon to `onload.sqf`. Events referencing the main GUI's `onload.sqf` resulted in the MSG icon missing.