Replace the existing undocumented tri-state 'try_order' parameter with
the boolean keyword-only 'preferred' parameter. Setting it to True
places the browser at the front of the list, preferring it as the return
to a subsequent get() call.

'preferred' is added to the documentation.

The traditional first entry in the tryorder queue is xdg-open, which
doesn't support new window. Make the default browser first
in the try list, to properly support these options.

…WatchdogEx

Fix issue python#85: don't schedule already scheduled tasklets.
Ensure that the internal watchdog list is empty before and after
all test cases of class TestNewWatchdog.

Fix test case TestNewWatchdog.test_watchdog_priority_{hard|soft}. The
timeout value was much to short to build up a list of watchdogs.

The new test cases TestNewWatchdog.test_schedule_deeper_{hard|soft}
triggers the assertion failure reliably. They are skipped for now.

https://bitbucket.org/stackless-dev/stackless/issues/85
(grafted from 2235702eb90cb0709dd40544b3952a956f2032a9 and 3fb55ce5b2d4)