[Extensions] Move kXWalkExternalExtensionsPath to extension switches

So that both xwalk_runtime and xesh (the test shell for extensions) can
use it. This prepares to make xesh build do not depend on the rest of
the runtime.

I was avoiding moving this constant here because it was not necessary
for extension system. But given the multiple users, I consider extension
itself a fair place to store this constant.
[Extensions] Make xesh depend only on what it uses

Currently xesh (XWalk extension test shell) depends on the entire
xwalk_runtime target. This is not needed, since the shell was designed
to use only Extension system.

One drawback of the current approach is that unrelated changes end up
forcing a recompilation of xesh, this have some impact when running
ninja without target parameters.

Instead of depending on the xwalk_runtime target, this patch makes xesh
depend directly on the base libraries and include the sources from
extensions.gypi.

This could be further improved by making extension system itself into a
static library, used both by xwalk_runtime and xesh. This would avoid
compiling the extension system files for each target.
[Extensions] Compile extensions as a static library

Since extensions shouldn't depend on other parts of Crosswalk, we can
compile it as a static library. One advantage is that now both
xwalk_runtime and xesh depend on that library, avoiding multiple
compilation of some source files.
Merge pull request #1335 from cmarcelo/extensions-as-static-library

[Extensions] Compile extensions as a static library