core: allow passing the cwd to fork()

Signed-off-by: David Aguilar <davvid@gmail.com>
cmds: add a command for launching a terminal

The preferred terminal can be configured by setting `cola.terminal`.

Signed-off-by: David Aguilar <davvid@gmail.com>
bookmarks: add context menu actions

Add new actions for launching a terminal, launching an editor, and
opening a repository using the OS-default application (e.g. a file
browser).

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