Add initial support for QtTesting

When testing is enabled new menu entries are available, we can pass in
an XML event stream on the command line and replay it. This currently
defaults to existing once replayed, and I added a sample XML event
stream to the data repository. Any XML in the directory are globbed and
added as tests.

We still need to add measurable artifacts, where a simple one for
Avogadro would be image differences or the active molecule after a set
of operations.

Change-Id: I95e35dc1d8cff9846305cfab6cb71780da62f8e5
Add a disable settings option

This helps with testing (always loading a known state), and when people
would like a simple way to reset everything back to the defaults.

Change-Id: I2c2221bca4902406289947bf1192c29cb7d9bde0
Set the AVOGADRO_PLUGIN_DIR for the tests

Ensure we look in the AvogadroLibs_INSTALL_PREFIX for the plugins,
guaranteeing we will find the correct ones even when running the tests
from the build tree.

Change-Id: I1cfce094aa12207eb10e3ddaa771c05bf9ac7679
Merge topic 'qt-testing' into master

bc38d716 Set the AVOGADRO_PLUGIN_DIR for the tests
df34bcee Add a disable settings option
d11c6740 Add initial support for QtTesting