Add support for pekwm

Add support for creating pekwm menus. The application menus can be
placed in a submenu or not, have icons or not and the XDG Path attribute
works.
The menus can be either static or dynamically generated.
Add support for pekwm dynamic menus

Add a --pekwm-dynamic command line option that allows using xdgmenumaker
for creating dynamic menus for pekwm.
Add a manpage section about pekwm
man page fixes

Fix spacing, turn several bold blocks to verbatim blocks and fix some typos.