Stable win32 toolchain based on MinGW for ownCloud Client 2.3

When creating a new stable branch:

- Make sure to copy the project config (Advanced -> Project Config)

- Copy all packages that are in this project from windows:mingw:win32, except for
  * Qt packages
  * mingw32-cross-nsis-plugin-uac
  Those need to be copied from this repo.

- run mingw_toolchain_check.sh from github.com/owncloud/administration/build-clients