database: ignore empty zone datasets

readme: add wotlk download and install instructions

database: replace taxi icons with game textures

map: use the game's ToggleWorldMap() to open the map

This ensures that the UI's layout handling code runs, allowing players to close the map with the escape key.

quest: add quest completion state to tooltips

database: make sure the quest-cache always exists

map: add quest objective text to tooltips

database: add description string for friendly units

database: only build unified markers on valid quests

database: check for existing locale first

Check for available language package first before
trying to access it. This could have thrown an error
for people who downloaded a enUS version of the addon
but playing on a frFR client for example.