GuiFont Concurrency Issue

[jgehrig]

There is an issue related to #868 when updating GuiFont and guifont.

See tst_shell.cpp, the issue can be demonstrated by removing the following line:

	QTest::qWait(500);

If a GuiFont command is sent too quickly on startup, the callback to update guifont can occur after the call to GuiFont is made. This triggers a mismatch between guifont and GuiFont and messes up the shellFontChanged signal.