Account for the margins used by Windows around status bar text.

Account for the margins used by Windows around status bar text.

Because Windows uses margins around the text drawn in the status bar, naively
setting a field width to the size of the text didn't work (see previous
commit for an example). As this seems a natural enough thing to do, account
for this margin inside wxStatusBar itself to avoid the user code the trouble
of having to call some special function to do it. Notice that this does mean
that fields not containing text may be slightly larger than needed, but we
consider that this (rarer) case is less important.

Also account correctly for the status bar grip size. And while we still hard
code its size, do it in a clearly named function instead of using completely
mysterious constants here and there.

Closes #10696.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61992 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775