Don't store rendering mode in Texture

If this is a property of the texture itself, that fact should be stored
in the texture metadata. If it's a property of rendering, we should note
that when it's rendered, as we do after this commit.
Add AssetManager

AssetManager owns textures and is responsible for loading them, or
loading the new 'missing' texture if the file is unavailable.
Use AssetManager in Terrain
Implement Texture reloading
Use inotify to monitor for changed assets
Merge branch 'asset_refresh' of ColdPie1/openage into inotify
terrain: moved hotspot fixing to convert script
texture: many member functions can be const
texture: stylistic fixes for comments, added spaces
texture: added this-> pointers and reformatted initializer list
texture: improved file suffix changing by strncpy
terrain: blending and terrain metadata vectors now const&
main: better warning message for the file uglyness
main: removed redundant asset loading check
main: added announcement log message before loading gamespec
assetmanager: added documentation in header
assetmanager: stylistic fixes, this-> and formatting
assetmanager: check for errors when interacting with inotify
assetmanager: use one missing-texture only
assetmanager: optimized hashmap lookup