Debugger: Improve tooltip for out-of-scope items with gdb

Previously, hovering over 'i' when stopped in main()

   int foo() { int i; }
   int main() {}

produced a tooltip without contents, now there is an
item saying "<not accessible>"

Change-Id: Id4de72541f74af93ba65c2a9508cf21e1dcd6569
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>