Fix reentrancy issues in wxMSW wxTextCtrl max length handling code.

The changes of r75940 didn't work correctly if the handler of wxEVT_TEXT in
some text control modified a (potentially) different text control, as the same
global variable was reused with disastrous results. Avoid this by keeping a
stack of insertion lengths instead.

Using a per-control field would work too, but would be a bit wasteful as the
size of the stack will rarely be more than 1 (and never much more) and this
change can also be applied to 3.0 branch without breaking ABI.

Additionally, fix another problem in r75940 which used 0 as a special marker
for the insertion length, which result in redoing each insertion of empty
string (which is another word for Remove()) twice unnecessarily, by using -1
instead.

Closes #15980.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76193 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775