Use VERSION file to manage DTK version. VERSION file defines package
version. It also decides the initial value of cached variable
DTK_VERSION which controls the project version in cmake build system.
VERSION file is not synchronized to DTK6, as DTK6 has its own VERSION
file. One can still pass version to DTK_VERSION when running cmake
configure. This is helpful for developers.
Signed-off-by: Yixue Wang <wangyixue@deepin.org>