Prepared variables for build directory transition.
Changed build directory creation timing.
Fixed dependency.

Added missing OASIS xslt icons into xsl/images.
Put latest GnuCash-specific icons into xsl/icons.
Removed stylesheets/.
Changed CMake files related new directory structure.
Changed ${BUILDDIR} to ${CMAKE_CURRENT_BINARY_DIR}/${fmt} for CHM, PDF, and EPUB target.

Removed xsl/1.79.2/ completely.
Removed unused customization files.

Fix target install for each format.
Clean up unused variables.

FO and HTML have different formats, so you need to customize them appropriately depending on how docbook-xsl is written.

Rewrote titlepage and variablelist for each format.

* US letter for C,
* A4 for other languages.

ISO A4 paper size is used worldwide but letter and legal size
are mainly used in the US. Therefore paper size is set above.
This setting should be moved CMake option in the future.

…pectively.

CHM format uses traditional Windows codepages, not Unicode. Therefore
traditional character code should be applied for the HTML and TOC.

CMake's clean feature isn't enough to remove some generated files.
Therefore, add clean-extra target which will delete unnecessary files that clean left.