[Therion] Therion wx3 support tweaks

[Olly Betts]

Patch attached to fix some lingering issues.

One is that wxGLCanvas::OnSize() is deprecated in 3.0 - the portable
(between platforms and wx versions) way to achieve this is to call
event.Skip(); which will cause the event to be propagated to the base
class.  AIUI, the only platform this matters on is OS X, which I can't
test on - it would be useful to confirm that resizing the window still
works on OS X.

The other was in my original patch, but got lost when it was applied.
As a result wx3 gives an "assertion failed" if loch fails to load a
file.  The issue here is that the wxWindowDisabler needs to be off
before the messagebox gets shown, which I had carefully arranged by
reordering code and adding an extra { } block around just the section
which needs protecting.  I've added a comment this time, to document the
reason for the extra { } block.

Cheers,
    Olly
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