Prompt to overwrite in Windows Save As dialog

Add OFN_OVERWRITEPROMPT so the dialog will prompt before overwriting an existing file.
Remove the OFN_FILEMUSTEXIST flag, which works only with Open dialogs.
"Enter in replace entry does Replace & Find instead of Replace" setting
in General/Misc/Search.
TM: simplify match test for current tag
TM: don't needlessly duplicate array when searching for current tag
Cleanup Plugin Manager dialog

Merge "Name" and "Description" columns, and move plugin details to
tooltips.
Add foreach_document(), documents[] examples
Fix sign comparison warning
Fix unsigned >= 0 comparison warning
Fail if Doxygen is enabled but not found
Merge branch 'ntrel/doc-macro-examples'
Fix doc/Doxyfile.stamp rule portability

$< is only portable in suffix rules, so don't use it there.
Remove unnecessary $(builddir) prefixes
Force enabling API docs on distcheck
Improve implementation of utils_str_equal()
Improve Docutils (and friends) support in Autotools

* Add `--enable-html-docs` and `--enable-pdf-docs` arguments
  (off by default)
* Add `RST2HTML`, `RST2LATEX` and `PDFLATEX` to override path to these
  tools.
* Add HTML and PDF doc status messages to configure summary
Use improved Docutils Autotools support

FIXME: The `install-html-local` rule might be wrong since it's
copying `$(srcdir)/geany.html` instead of `$(builddir)/geany.html`
because for some reason generated `geany.html` is checked-in to
version control in the source `doc` directory.
Add `--enable-html-docs` to distcheck flags

FIXME: `--enable-pdf-docs` not added because I can't make it pass
distcheck due to quirks of `pdflatex` (see comments in
`doc/Makefile.am`)
Unconditionally install HTML manual (oops)
Replace autoconf directory variables with sed (Windows makefile)
Fix document pointer documentation

Improve clarity and remove wrong documentation about using `is_valid` to
check if a document still exists.