SYM: Workaround a bug when VKB covers text input

Closes #32 - Text input is covered by VKB in landscape mode.
SYM: Disable header when VKB is active
SYM: Fix an issue when VKB state wasn't preserved

When having VKB open, switching to another application and returning to
TAO translator, VKB was hidden while it should remain in the open state.
SYM: Workaround a problem with VKB

When having VKB open, switching to application with VKB open and
returning back, `inputContext.visible` is set to false while VKB is
still visible. This seems to be a bug in Qt or even lower.

The workaround assumes that when Symbian reports that application window
is partially visible and text input area is focused, then this is VKB
that partially covers the window.