Now all C++ files need to #include <util/prec.hpp>
 That is why all .cpp files are touched by this commit

Many changes to installers and update checking:
     - the window is now called PackagesWindow, in a new source file
     - update checking is now independent from the PackagesWindow. For update checking only a list of package versions are needed (vector<PackageDependency>). This is much less information to download at each startup.
     - the list of available packages is now a list of available Installers, since an installer can contain multiple packages.
     - moved the logic of dependency checking etc. to data/installer
     - moved the actual installation to util/io/package_manager
     - moved directory iteration/creation logic to util/file_utils
     - added PackageDirectory: the local and global package directory now have their own object (was part of PackageManager)
     - added PackageVersion: for detecting if a package has been modified after it was installed.
     - added PackageDescription: description/header of a package. Basicly the same as what Packaged provides.
     - added DownloadableInstaller: where to find an insaller, what does it contain?
     - added InstallablePackage: brining it all together: installer, package, status, action.

Current status: the insaller is currently broken in a few places, more on that soon.

 - Some refactoring of the other field types

 - common code of ChoiceValueViewer and MultipleChoiceValueViewer put into functions
 - RotatedDC can now draw text with shadow.
 - DECLARE_STYLE_TYPE macro and friends do slightly more.

Optimization: removed the conversion via wxImage for downsample_to_alpha (profiling indicated it was a bottleneck)

 - Optimization: common parameters to built in functions are no longer looked up as a string at each call, instead their integer ids are global constants
 - Optimization: some other minor tweaks.
 - Nicer --help message

Major changes:
  - when rotating, the top left corner of the rectangle stays in place.
  - ValueViewers get a dc that is pre-rotated/translated for them, i.e. (0,0) is the top-left of the viewer (with ValueViewer::getRotation).
  - moved 'angle' from individual Styles to the Style base class.
  - any rotation angle is now possible. angle is still an int for now.

This warrants a version bump

  "depends on: package version"

Added a 'tree list' control, which will be used for the package/updates window to be able to group packages

made update_checker compile in vc

Added infinity mana symbol.
Added insert symbol menus to all symbol fonts that were lacking them.
Updated font color choice for fpm promo to improve readability.

Corrected a mistake in extended image styling for magic-new and magic-planeshifted.

Adjusted fields for magic-planeshifts and alignment in magic-new-promo.
Added transparent mana symbol watermarks.

NOTE: I removed the default list of creature types. I don't want it 
gone, I just found the update easier to make without a simple list. If 
someone else feels like making one themself, I don't object.

Updates now work, and have been tested.
Made a few other updates.
To work on: aesthetics?