Makefile: Set PHONY targets using variable

This patch ensures that the .PHONY target is assigned only once using a
list of targets assigned to the variable $(PhonyTargets). This prevents
warnings about multiple definitions of .PHONY.
Add "make todo" and "make fixme" targets

The "todo" and "fixme" targets search the available source files for use
of the TODO: or FIXME: tags. Example use:

$ make todo
./sec/introduction.tex:14:% TODO: how big are these optimisation spaces?
./sec/introduction.tex:23:% TODO: this last sentence is weak ^^
./sec/introduction.tex:29:% TODO: the case of self-tuning systems: TCP/IP.
./sec/introduction.tex:66:% TODO: parameter convergence time?
./sec/methodology.tex:26:% TODO: is this next sentence necessary?
./sec/methodology.tex:33:% TODO: What are these challenges? ^^
./sec/background.tex:26:% TODO: good for unknown inputs.
./sec/background.tex:67:% TODO: one more paragraph to fill the page
./sec/background.tex:138:% TODO: reword this week last sentence ^^
./sec/background.tex:140:% TODO: one more paragraph to fill this page

Both "todo" and "fixme" targets have the same behaviour.
Makefile.latex: Add DocumentSources to Sources

This adds the $(DocumentSources) list to the full list of $(Sources).
Some proposal work