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Translated using Weblate (German)

Fix typo
Translated using Weblate (Portuguese)
Add minetest.get_gametime() API function, that returns the number of seconds since the world was created.
Add fallback font support for some languages.
Run updatepo.sh
Add dummy and LevelDB database backends
Make it compile
Add message that LevelDB is not supported on Windows
Use STL containers instead of irr::core::list
Fix 'Unknown map backend' error when using Configure menu right after creating world
Add note about --migrate only working with minetestserver or --server
Make --migrate update world.mt too
Prevent ModMgr from deleting backend setting in world.mt
Use system-wide LevelDB instead of bundled one
Change mainmenu texture handling + small misc changes

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 formspec table and reorganize formspec helper docs a bit

Also rename engine.get_textlist_index to engine.get_table_index