keys: remove keyboard configuration from Lua code

Everything is now handled by inputplug.
keys: fix custom symbols for X1 Carbon
start: don't try to start xcompmgr

compton takes some time to start and meanwhile, xcompmgr will be
started...
keys: don't use `replace keys` when defining new symbols

It seems that this is incompatible with a per-device mapping.
urxvt: use a more transparent background by default

On a HiDPI screen, the previous transparency was almost not transparent.