added help commandline option to explain basic usage

Show basic usage when -h or --help is specified as commandline options. I also
changed window id conversion to use QString functions. As before a 0x prefix
specifies a hex number, otherwise it's interpreted as decimal number.
made path to dmenu configurable

cmake now searches for dmenu and one can also set DMENU_PATH manually in cmake.
If cmake can't find dmenu, DMENU_PATH is set to "dmenu" which means that dmenu
needs to be in PATH when qmenu_hud is run.