Cleanup Plugin Manager dialog

Merge "Name" and "Description" columns, and move plugin details to
tooltips.
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.
Fix distchecking with PDF enabled
Don't truncate pdflatex warnings
Enable PDF generation on distcheck
Merge branch 'ntrel/fix-valid-doc-dox'
Waf: correctly link libm as required
Adding while to list of SQL keywords
Merge pull request #249 from b4n/b4n/waf/link-libm

Waf: correctly link libm as required
Try to fix some duplicated accelerators inside Edit->Commands
Update tooltip string for check_cmdline_new_files
Update open string on filebrowser plugin to make clearer, that the file will be opened in Geany
Hide column headers in plugin manager dialog

They are not very useful and waste both vertical (the headers
themselves) and horizontal (first column wider because of title)
screen space.
Merge branch 'plugin-dialog-cleanup'