Build HTML documentation automatically during the build process

This is an improvement as requested in #322 to update the HTML docs
after geany.txt or geany.css have changed.
Also, with this change the resulting HTML will be generated in the outdir,
not in srcdir/doc anymore.
Update Waf to 1.7.16
Add "hackingdoc" as proper action and remove non-working --hackingdoc option
Add option --disable-html-docs
The missing bits for HTML docs: only on GIT build and support in-tree geany.html
Explicitly specify extensions when searching for rst2html

This is cleaner than searching for rst2html and rst2html.py since Waf
offers the API for this and this change enables Windows support.
While at it, add binary name 'rst2html2' for ArchLinux support.
Reuse ConfigurationContext when searching for programs if we have one

This enables proper logging when checking for rst2html during configuration.
Cleanup apidoc and hackingdoc commands to use ctx.top_dir and ctx.out_dir

This should be cleaner and safer than using '../' mixed with os.chdir().
Fix HTML documentation installation for VPATH builds
Don't try to install the HTML documentation if we don't have it
Update Doxyfile
Document document_find_by_id() parameter so Doxygen is happy
Merge branch 'doc/untrack-geany.html'