[imp] Add the option to set the default language to the active language

This adds the option to set the default language for a language field to the default frontend or backen language. Basically it adds the option to use:

<code>default="site"</code>
<code>default="administrator"</code>

It also allows devs to store a dynamic reference to the active language. For exampe if you want to always store the active frontend language you can save "site" and you always get the active language. Without this you would, for example, store en-GB and then if the language is changed you will keep getting en-GB.

Example loading of frontend active language:
```XML
			<field name="language" type="language"
				client="site"
				description="COM_USERS_USER_FIELD_FRONTEND_LANGUAGE_DESC"
				label="COM_USERS_USER_FIELD_FRONTEND_LANGUAGE_LABEL"
				required="true"
				filter="cmd"
				default="site"
			/>
```

Example loading of backend active language:
```XML
			<field name="language" type="language"
				client="site"
				description="COM_USERS_USER_FIELD_FRONTEND_LANGUAGE_DESC"
				label="COM_USERS_USER_FIELD_FRONTEND_LANGUAGE_LABEL"
				required="true"
				filter="cmd"
				default="administrator"
			/>
```

You can reference frontend active language with:
<code>'site', 'frontend', '0'</code>

You can reference backend active language with:
<code>'admin', 'administrator', 'backend', '1'</code>