1.2 M5E2: Change module category Global to System

All modules are assigned to a category in their module info, usually one 
of Global, Content, Miscellaneous, or Users & Groups.

This category is typically where the most crucial modules in Xaraya are 
assigned. Most core module are assigned to the Global category, and many 
other non-core modules assign themselves to it.

This change renames the Global category to System, in order to more 
clearly express the significance of the modules within.

For 1.2, module developers are encouraged to adopt this change in their 
Global modules. If not, the system will automatically make this change 
upon module install.

See also: http://www.xaraya.com/index.php/xarbb/topic/3440