qmake|libgui|Scripting: Moved gui.de to libgui

gui.de directly relates to functionality in libgui, so it should be
located in libgui's modules.

Project files updates accordingly. client.pro still handles the
installation of all Doomsday Script modules for the Doomsday client.
SDK: Added a .pri files for apps using the SDK

doomsday_sdk.pri is installed to the SDK directory.
Fixed|libdeng2|SDK|qmake: Missing public header
libdeng2|Unix: Expand paths specified in UnixInfo config

Any symbols like "~" found in the paths of UnixInfo config files are
automatically expanded.
libdeng2|App: Using a .pack as basedir; configurable config path

This commit adds flexibility to App. It is now possible to point the
application's basedir to a pack file, so that the contents of the pack
are used to feed the App's root folder. In this case, the usual /data
and /modules are expected to be found inside the pack.

Also, one can now specify a different path where Config reads its script
from (defaults to "/modules/Config.de" as before).
libdeng2|Cleanup: More information in error messages