There were some missing headers like <stddef.h> for `NULL`, <wayland-util.h>
for `wl_link` and <stdbool.h> for `true`/`false`.
Also this commit fixes that `labwc.h` and `ime.h` included each other.
Made all header files to have LABWC_ prefix in include guard identifers.
Converted from __LABWC_ in 35 include/ files.
Converted from __LAB_ in 5 include/ files.
Added LABWC prefix to 3 include/ files.
Added include guards to 3 include/ files.
The double underscores were removed since according to C standard
those "are always reserved for any use".