added ability to hide icons with Modkey+n keyboard shortcut

Unfortunately only one client can use X_GrabKey at a time for a particular
keyboard shortcut combination, therefore Modkey+b cannot simultaneously
hide the taskbar for DWM and hide the icons for thingmenu. The applications
must be merged for this to work.

config.h
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- added Keys array and defined keyboard shortcut

thingmenu.c
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- added X11 keyboard library
- added Arg and Key typedefs
- added keypress, togglebar, grabkeys, updatenumlockmask functions and function declarations
- re-arranged event array alphabetically (to match DWM)
- added icon_width and icon_height to record the starting width and height of the icons
- added XUngrabKey to cleanup function
- added grabkeys() to the end of the setup function