Add -Wshadow to cflags

This is one part of issue #5.

mentioned in merge request !129 (closed)

@anteater looks good to me code wise (and it seems wlroots copes as well now - which was one of the reason this wasn't turned on earlier). Thanks for hanving a look!

Could you split into 3 commits so e.g. reverts and bisects stay simple:

• one fixing up keybindings.h (since this is a real bug declaring a seat variable which is never used)
• one fixing up the cursor.c (this is an actual -Wshadow case)
• the last one turning on -Wshadow (this flips the switch to not regress)

i'm then happy to merge this.

@anteater ping? Otherwise i can make that split if you're busy otherwise.

mentioned in merge request !134 (merged)

resolved all threads

closed