Unified Test App

test_app.cpp now takes a single argument specifying the desired backend,
ie:

$ test_app sdl
$ test_app dfb
$ test_app fbdev

Since some of these may not have working input event_systems as yet, the
app will automatically quit after 30 seconds using Marc's mechansim from
test_app_dfb.

If the owners of the other test_app_*.cpp files are happy with this
approach, please remove your test_app_*.cpp file from the repository and
update the Makefile.am appropriately.

This reduces the number of test_apps to fix when the API changes
slightly and greatly reduces build time of tests. (Once the other tests
are removed).

Darren