If dmenu didn't return anything we returned out of the function without
calling `waitpid` to collect the childs exit code, leaving to the child
process to be left as a zombie indefinitely. This commit makes `waitpid`
be called before the return checks are done.
When generating the list of urls to pass to dmenu, string_append with a
newline as a separator was called unconditionally. This caused a newline
character to be added for each notification even if it didn't contain
any urls, leading to empty items in dmenu.
Fixes#300
string_to_argv parsing was very simplistic and didn't properly handle
quotations. Since we are already using glib, g_shell_parse_argv serves
the same purpose with much better parsing and error handling.