Use smart pointers to protect access to TabBoxClient

Client holds a SharedPointer to the TabBoxClient and only
provides access to a WeakPointer which is passed to TabBox.
ClientModel is adjusted to hold a list of WeakPointers instead
of the direct pointers.

This fixes the following reproducable crash:
1. Configure both primary and secondary TabBox with different
   layouts
2. Use primary TabBox
3. Close a window, best the one which used to be active
4. Use secondary TabBox
-> Crash

The reason is that the ClientModel still contains the pointer
to the deleted TabBoxClient in step 3 and while creating the
layout access to the TabBoxClient is needed to get the Client's
icon.

By using the weak pointer it can be ensured that we don't try
to dereference the deleted pointer and prevent the crash.

Cherry-Picked from 05a34201

BUG: 290482
BUG: 285747
CCBUG: 237345
FIXED-IN: 4.8.4
REVIEW: 105000
REVIEW: 105069