Fix Save Preview As... for textures, now save as any type we can encode to!

Removes the previous method and condenses the menu entries back into one.
 - Translations have been updated except for German, German will need to update the translation for "Save Preview As..." since the name hasn't changed.
Adds FFSAVE_IMAGE to the filepicker enums
Crossplatformity:
 - The Windows version works nicely
 - The Linux version may not work or even compile but it should..
 - The Mac version hasn't been added, instead it will default to accept any(all filter), should the provided file extension be invalid a notification will display.

indra/plugins/filepicker/llfilepicker.h

@@ -113,6 +113,7 @@ public:
 		FFSAVE_AO = 35,
 		FFSAVE_BLACKLIST = 36,
 		FFSAVE_PHYSICS = 37,
+		FFSAVE_IMAGE = 38,
 		// </edit>
 	};