fixup plugins loading sequence
add "describe_plugins()" to the application lua kernel
add some debugging output to plugins manager on channel "plugins"
change "get_str" plugin callback shim to make a temp string copy

This prevents a segfault when using "send chat message".
add some debugging output to check that plugins context is running
add error reporting to try to understand wesnoth.require error
fixups: don't construct string from NULL, avoid complicated API

In the error handling routines for compiling and running a script
at runtime, don't construct a string directly from lua_tostring,
because if it is null that is a runtime error. Instead check for
this scenario.

In intf_require, when getting the "wesnoth" global, luaW_getglobal
is unnecessary and lua_getglobal should be prefered.
bugfix: intf_require would segfault if a thread called it

The problem was that intf_require would assume use the wrong
lua_State* since it finds it using the kernel's this pointer. This
commit gives a version of "protected_call" which allows to use the
callback's lua_State*.
fix a binding for "select_game" for the plugins context
fixup multiplayer lobby plugins bindings
add "get_int" shim, similar to "get_str"
add an automatic plugins context to all gui1 dialogs
bugfix: events were being discarded before the plugins got them
add notification of chat events to all mp ui windows
add a plugins context to mp create
add "get_selected" accessor to mp create plugin context
add "find_level" accessor to the create engine
add a "select level" callback to mp_create
add a plugins context to mp configure
add a plugins context to multiplayer connect