Aruba 1.2 Milestone 3: Template extension change

These changes have been completed in the just-pushed aruba.xd2xt scenario.

All templates have been renamed from *.xd to *.xt.

Template handling has been adjusted accordingly.

Support for .xd files has not been removed, only deprecated.  If an .xt 
file is not found, the corresponding .xd file will be looked for before 
a "template not found" exception is thrown.

This change affects module development (not themes) going forward. 
Module developers can break compatibility with 1.1.5 and below by simply 
renaming their templates.  If module developers choose to do this, they 
are strongly encouraged to increment the minor digit of the module's 
version number by 1, eg, 1.2.x to 1.3.0.

For now, compatibility with 1.1.x and 1.2+ can be maintained by shipping 
xd templates only, or adding identical xt files which will be preferred 
by 1.2.

This scenario should be complete, but needs thorough testing before 
being merged.

See also: http://www.xaraya.com/index.php/xarbb/topic/3432