Texture names must now be escaped in formspec elements image[],
background[], image_button[], image_button_exit[].

Instead of special-case handling of texture loading (and unloading
which was missing) in guiFormSpecMenu.cpp, use the newly created
ISimpleTextureSource interface which is a minimal subset of
ITextureSource. There is an implementation of this interface
used by GUIEngine (MenuTextureSource).

Fix an off-by-one bug in unescape_string; it caused requests for a
texture called "\0".

Add engine.gettext() and remove gettext() calls in guiFormspecMenu.cpp

Fix params number

Add pressed texture name

Fix string def

Fix syntax error

Fix RUN_IN_PLACE=0 mod install (now mods are saved in user folder in that case)
Fix text of field element not unescaped correctly

This reverts commit 39d03148.

Always use address for server
Fix first (empty) element beeing selected after deleting last world

The previous code did not work when the list did not start at index 0
of the inventory list (issue #779).

Fixed also game bug drawing dragged item behind fields, buttons etc.

by using the cursor coordinates directly, instead of updating them only when a mouse event is seen.