x11-wm/obpager: modernize, undeprecate, take maintainership

• Avoid C++11 narrowing issue altogether by not truncating displayed workspace numbers so long they fit 4-char buffer
• Convert GNU sed(1) to more standard extended regex syntax
• Clean up the Makefile patch; nuke ugly `post-patch' target
• Adjust LICENSE (GPLv2 or later), unfold description text