Allow user to edit open dialog filename when file doesn't exist

This should be cleaner and safer than using '../' mixed with os.chdir().

This enables proper logging when checking for rst2html during configuration.

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.

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.

Closes PR#324.

Closes PR#322.

Support pseudo-unique IDs for documents

Properly handle existing session when creating new project

Occurred on Project->New with an existing project when closing an
unsaved file was cancelled.

Also rewrite the default session in case it was cleared.

e.g. used to 'print' debug output to log on e.g. Sybase ASA

Clean the HTML manual upon 'maintainer-clean' rather than 'clean' in
case it was not built by Make but rather part of the distribution.
This is fine even then, as configure will properly require what is
needed to build it again if it is missing.

Don't set clean-local and all-local dependencies all together at the
end and rather let each section add the appropriate dependencies.
This makes each conditional more self-contained, and is safe as Make
allows adding extra dependencies to existing targets.

Enable building of the HTML manual by default unless not building from
Git and with an existing local copy (which is included in distribution
tarballs).

This makes sure we can install the HTML manual without having it
checked in VCS, yet not require rst2html for tarball builds.

Ask whether to adopt the existing session documents.

Also fixes the existing bug of not reloading the default session when
an existing project is open and a new project is cancelled.

As it is generated by the build system it doesn't have to be checked
in, and having it in became a whole lot more annoying since it started
being generated automatically on `make all` instead of specific (and
weird) `make doc`, as it means whenever the documentation changes the
HTML is re-generated on any make run.  This is particularly problematic
if using a different Docutils version than the one used to generate the
checked-in version as it would create local noise that should not be
committed, yet keep being annoying.

This situation probably leads to most people disabling (or at least,
not enabling) the documentation generation on normal builds, itself
leading to more hassle updating of the documentation.

Add GeanyDocument::id, document_find_by_id() to plugin API.

This also fixes clicking on a Messages item whose document has been
closed and reused. Now the click will be ignored instead of jumping to
an unexpected line in the new document.

Adding new keyword hold, used e.g. on SQLAnywhere to open a cursor 'with hold'

From a patch by TJF.  Part of [feature-requests:#691].

Make reflow paragraph leave cursor at end of reflowed text, which is more likely to be useful than where it was left at the start of the last line.