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
Draw version in lower left corner, take 2

 * Version drawing and FPS drawing now done in the same
   function, toggled by F3.
Add a screenshot manager, and let it save the screenshots with a nice name
Merge pull request #62 from franciscod/screenshot

screenshot: refactor with numbering and timestamping
Merge branch 'draw_version' of https://github.com/oldlaptop/openage into oldlaptop-draw_version

Conflicts:
	copying.md
	cpp/engine.h
doc/editor: initial ideas for the map/campaign editor
engine: removed 'version' text from version overlay
Merge branch 'asset_refresh' of ColdPie1/openage into inotify
use $(MAKE) variable in main Makefile

Using special $(MAKE) variable has some benefits over explicitly calling
'make'.
When using this variable, make knows automatically to pass some of its
arguments to lower-level invocations.
Among other features, this enables multi-threaded building from top-dir.
Merge pull request #78 from janisozaur/make-fixes

use $(MAKE) variable in main Makefile
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&