Made all keybindings reconfigurable in `config.h`.

Now key bindings are not configured via switch/case but via an array of
`Key`s. `Key` is defined in `event_handler.h`.
Added `event_handler.(c|h)` to Makefile.
Moved most of the layout conf into `config.def.h`.
Merge branch 'master' into enhance_config.
Fixed broken scrolling in exif column.
Merge branch 'master' into enhance_config.