Make test base a self contained library

So browsertests binaries can link with it. Previously it was part of the main
xwalk_browsertest and had the assumption of a single test runner.

The ideia is to split the browsertests by components, making the faster to run
and also helping to keep track of dependencies.
[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.

- Have separated test runners for extensions, making run tests faster.
[SysApps] Buildsystem refactoring

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

Filter files suffixed with _tizen or tizen/ directories, depending on
whenever 'tizen' or 'tizen_mobile' is set.For now on, let's stop using
conditionals for source code in our build system.

The rules will automatically apply to every project file, because common.gypi is
included by gyp_xwalk script.
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.