Debugger: Consolidate GDB and LLDB "updateLocals" code paths

This splits the bool setToolTipExpression() operation into
a bool canHandleToolTip(), and the actual processing of
the request, which is mostly identical to the handling of
a watcher.

Handling a watcher is now mostly the same as a full Locals
update, except for the 'partial' flag. Pushing the handling
of that down to the bridges gives identical code paths
in the gdb and lldbengine. Move that to the
DebuggerEngine base class.

Change-Id: I3861b43e8630c7e7bd57fcd549b2a2387e3d4869
Reviewed-by: hjk <hjk@theqtcompany.com>
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>