[virtualkeyboard] Fix the qtvirtualkeyboard with Qt 5.15

On 5.15, the VirtualKeyboard did not have a correct z-order which caused
the virtualkeyboard to hide as soon as key was pressed, upon
investigation it seems that upstream Qt changed behavior in following
changes.

https://codereview.qt-project.org/c/qt/qtvirtualkeyboard/+/292472
https://codereview.qt-project.org/c/qt/qtvirtualkeyboard/+/292674

Now when desktop-integration is used (i.e non-application embedded mode)
one need to set property desktopPanel to true so that it have correct
z-ordering and does not miss input events.

This is used in Component.onCompleted to avoid breaking it for the Qt
5.14.x qtvirtualkeyboard. When we switch to 5.15 property assignment can
be moved to InputPanel declaration.

(cherry picked from commit 28d5a4e7)