Provide support for regular Super/Windows key press

[gothka404]

Case/reproduction:
The Super-key in Debian or GNOME-based desktop environment is quite important - a single regular key press leads to "Activities Overview" as central navigation hub with search capability.

AVNC supports Super-key via virtual keyboard. It acts as modifier/toggle - if I press virtual icon, it gets focused and can be combined with other keys (long pressing appears to have the same effect). Another press will untoggle Super-key.

What seems to be currently missing is the ability to press Super-key as regular key to activate said Activities Overview.

Suggestion:
Forward regular character pressing of Super-key. This might be enabled by either long-tapping or double tapping.

Workaround:
Settings -> Input -> Key mappings -> Enable "Right Alt -> Super"
This will allow to double tap Alt to cause a Super-key press. Unfortunately key combinations like Alt+F4 are not possible with this change.

Log:
With regards to log, you can have a look at #256 (comment) again (same environment). Additionally I tested with Debian + GNOME + X11 + x11vnc, which has same issue.

Let me know, if you need more info!
(I wasn't able to do a Key test, as this seems only available for regular keys on Android keyboard.)

Related:
#141 #116 #99 #117