EWMHBaseStruts/EWMHNumberOfDesktops: per-monitor

Introduce helper functions, set_ewmhc_strut_values(), and
ewmhc_desktop_values() which can set the per-monitor base struts.  Switch
CMD_EwmhBaseStruts() and CMD_EwmhNumberOfDesktops() over to using them,
respectively.
parsing:  Update ABNF to reflect BaseStruts/NumOfDesktops change

Update the grammar of the EWMHBaseStrut/EWMHNumberOfDesktops command to reflect
the change in grammar.