Combine Style and Extension Management into one Customisations tab in the ACP

Currently, the Styles management has its own tab for only two 3 modules. The extensions manager that was recently merged is hidden away in the middle of the list of modules in the Systems tab.

I propose merging the two, as well as Language Pack management, into a new Customisations tab. This makes it easier to find these modules, as well as making the index more intuitively organized.

From the RFC:

• This would keep us from having related modules in unrelated categories. I do not see how Extensions relate in any way to the other items in the System tab.
• The Styles tab currently only has 3 modules, which is almost not enough to give it its own tab. Renaming it and adding another module or set of modules would give it more purpose
• IIRC, we plan on eventual integration between the ACP and the Customisations Database for easy remote installation of extensions and/or styles. By grouping them together now, we make that integration a more fluid process because it's all in one place.

RFC Link: https://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=43257