cmds: simplify things by removing a class

RunCommand exists so that we keep a reference to QRunnables.  Not doing
so invokes the wrath of the python gc and crashes some versions of
PyQt4.  Tuck the reference into the AsyncCommand class so that we can do
without the RunCommand middleman.

Signed-off-by: David Aguilar <davvid@gmail.com>
guicmds: refactor new_repo() into a reusable function

Signed-off-by: David Aguilar <davvid@gmail.com>
main: add a "File -> New Repository..." menu action

Signed-off-by: David Aguilar <davvid@gmail.com>