Provide virtual keyboard support on desktop platforms

There is currently no support on desktop platforms for the virtual keyboard. It is a ConditionalFeature that returns false, by defalut, on all desktop platforms. It can be enabled by setting an undocumented system property, "com.sun.javafx.virtualKeyboard" to either "javafx" or "native" (the latter is ignored on desktop platforms and treated the same as setting it to "javafx"), but it isn't supported.

This issue can be used to track the overall feature of providing virtual keyboard support on at least a subset of desktop platforms. It can be linked to related issues. I note that there are no concrete plans to implement this at this time.