Add play and pause commands

Add commands that always play and always pause, in addition to the
current toggle pause command.

Closes #342.
Merge remote-tracking branch 'promy/master'

Conflicts:
	src/settings.h
	src/ui_act.h
	src/ui_dispatch.h
makefile: use ":=" when using $(shell pkg-config...) to avoid duplicate evaluations

This speeds up the make process.