page add form reordering & language improvements

This pull request applies further improvements to the page add form:
![unknown](https://cloud.githubusercontent.com/assets/2105496/2766974/eeac5450-ca33-11e3-998c-bf8dd1d1b48c.png)

Furthermore, I've reordered the language file. The acp related language variables, prior all grouped in `cms.acp`, now are grouped more precisely by their general application area (e.g. `cms.acp.page` or `cms.acp.content`)