Patch: database version check

Hi, Dave

very simple patch:

pgAdmin3.h
+const short SERVER_MAX_VERSION = 0x0805;//Exclusive

pgServer.cpp

+ if (conn->BackendMinimumVersion(SERVER_MAX_VERSION >> 8, SERVER_MAX_VERSION & 0x00FF))
+ wxMessageBox(_("Connected to newer database using old version pgAdmin, some function could not run properly.\nUpgrade is recommended."));
+

I had tried to define a new class pgCheckBoxEntryDialog, but
it make pgAdmin's process always live after close.
I'm not familiar about it. First, make a tiny patch.

It's not needed to define a new function to check version.
So, define a exclusive version: SERVER_MAX_VERSION.

Before commit, don't forget to modify message text.

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Quan Zongliang
quanzongliang(at)gmail(dot)com
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