Caret is gone when switching to a VCL control via `TAB` and back to the FireMonkeyContainer

[yonojoy]

I think, this has a different cause than issue #12, therefore I added this as an other issue.

Steps to reproduce:

• Compile and start the demo applicaiton
• Click into the FMX edit field
• Enter a key
• Press the tab key
• Press SHIFT+TAB to return to the edit --> caret is gone

If I add FFMXForm.Active = True to the DoEnter function of FireMonkeyContainer, this issue seems to be fixed. But I do not fully understand yet how the FMX form is activated and deactivated...