Added input/module

This commit adds a new module that listens to file changes in the /dev/input/
directory. It also lists the initial devices.
Added ws_input_init to the main.c file
Merge pull request #273 from TheNeikos/add-preliminary_evdev_discovery

Add preliminary evdev discovery
Add CMake modules for finding xkb common
Use xkb common in CMake setup
Merge pull request #278 from neithernut/add-cmake-xkb_common

Add xkb-common to build setup
Add query for whether an object is an instance of a type
Add ws_transaction_push_statement() interface definition
Add implementation for ws_transaction_push_statement()
Merge pull request #286 from matthiasbeyer/add-transaction_functionality

Add a possibility to push a statement to a transaction
Merge pull request #283 from neithernut/add-object-is_instance_of

Add query for whether an object is an instance of a type
Add deserializer callback interface
Add deserializer callback source file (stubs) and CMakeLists.txt entry
Add implementation for NULL callback
Add implementation for boolean callback
Add implementation for integer callback
Add implementation for string callback
Add implementation for start_map callback
Fix: Name of a command must be assignable.
Add implementation of map_key callback