[Extensions] Buildsystem refactoring

The main point of this patch is to move build units from "includes" to self
contained modules. The reasons are:

- Includes have their path relative to the file which includes the .gypi. So if
another component not located at the root of xwalk depends on extensions
(sysapps for example), it won't work.

- Make it easier to track dependencies. Currently we don't really know what
dependencies we are getting for free from xwalk.gyp which is getting bloater and
bloater.
[SysApps] Make a self contained module out of sysapps.gypi

For the same reasons stated in the previous patch.
Add filtering rules for Tizen

Include this file other than src/build/filename_rules.gypi if you have files
suffixed with _tizen or tizen/ directories. For now on, let's stop using
conditionals for source code in our build system.
Remove platform conditionals for source code

Now that we have file filter rules for Tizen, we don't need to use conditionals
on the build system for source code we own.