Allow using ESC as accelerator in wxMSW again.

This ended up being broken due to an interplay between different unrelated
changes (at least r15120 and r41134) which were both correct, but didn't work
well together and resulted in not only preventing IsDialogMessage() from
handling ESC, but also our own accelerator tables.

Fix this by doing the check for IsDialogMessage() brokenness in
MSWProcessMessage() itself, just before calling it, instead of doing it in
MSWShouldPreProcessMessage() which is (and must be) called before
MSWTranslateMessage() which checks for accelerators using ESC.

Closes #3813.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77071 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775