Provide CMake build script

Require minimum of CMake 2.8.8. Allows use of OBJECT library type
in future (e.g. integrating TextInput in another project).

Move all headers/srcs to textinput subdirectory so that headers are
picked up using correct path.

Implement trivial CMake script to build TextInput library. Divide
headers/sources into public headers and private headers/sources
with the latter including the platform implementation details.
Build library using default type.
Correct internal links in README