Add trigger for Virtual Keyboard on long press of Middle Mouse Button

These commits add the ability to open the virtual keyboard by holding the middle mouse button for more than 1 second.

Though the virtual keyboard is not enabled by default on desktop ports, it is useful for some users who want to use it on their own compiled Win32/Linux builds, either as they want to use just a mouse/pointing device or they have mobility issues making the use of a keyboard difficult. 

This change removes the need for a hardware keyboard to press F7 to open the virtual keyboard in these cases, which has seemed fairly silly as the entire point of a virtual keyboard is to replace the hardware keyboard.

A survey of the game engines shows that the middle button is only used in a small number of engines and even then it is rarely used i.e. used in LURE to skip dialogues and in SCI to switch between walk and current action. By making the virtual keyboard trigger, a long press of more than 1sec to open this, it is very unlikely that this will cause any conflict, even if the middle button is used.

There is also a small cleanup commit here to tidy up the warnings emitted by the virtual keyboard if the pack is not present and prevent this being "noisy" on the console.

Does anyone have problems with this being merged? Should another commit be added to remove the F7 trigger altogether in favor of this new trigger?