Added [elseif] support, Lua part. Also included sanity checks for missing [then] tags inside [if]
Added [elseif] support, C++ part. This includes the missing [then] checks
[while]: added sanity checks for missing [do] tags
Get rid of obsolete if_while_handler function
[switch]: implemented checks for missing [case], variable= and value=, Lua part
[switch]: implemented checks for missing [case], variable= and value=, C++ part
Changelog entries for my series of commits
Merge branch 'elseif_branch' of github.com:wesnoth/wesnoth-old into elseif_branch

Merging the branch with the one I created locally.
Merge pull request #137 from wesnoth/elseif_branch

Implementing [elseif], plus sanity checks in ConditionalActionsWML