Add i18n support

I've already started work on this, awaiting further information after a call for action to the t-h.o mailing-list.

Most of the work done by now, but this is still an enhancement, right?

Of course I'll go and release changes soon, after I've been awarded to maintain this plugin by now.

Been quite some time since I've started that work, not dreamed to do so many other things to this plugin, before I'd be finally able to add i18n. But this is point in time approaching at least now.

(In [10021]) TracFormsPlugin: Add basic code and markup for i18n support, refs #8258.

This is accompained by a code cleanup especially aimed at Python docstrings. An ancient typo in cmd_page of TracFormProcessor has been fixed as well.

(In [10022]) TracFormsPlugin: (i18n) First message extraction, refs #8258.

Unified and shortened related msgids of TracForm? exceptions a bit. I18n markup has been tested with the German translations included here.

(In [10023]) TracFormsPlugin: Fix regression introduced by changeset [10021], refs #8258.

Custom submit_label values have been overwritten by optionally localized default label before.

(In [10079]) TracFormsPlugin: Add basic history browsing capability, refs #3445, #4759 and #8258.

Fix code around TracForms db handling after using tuple for 'src'. Plugin now contributes to parent resource's context navigation, db interface is grown by a method for reverse-lookup of forms and the 'Form' resource is getting extended to provide more information too, everything needed to establish infrastructure for history browsing. With the addition of templates we need to extend i18n capabilities as well. This is the base for later adding form reset and delete/purge functions.

(In [10080]) TracFormsPlugin: Partial msgid and translation update, refs #8258.

Msgid changes are done to finally unify similar messages. While extraction from templates is working, content is still omitted for being too preliminary by now.

(In [10093]) TracFormsPlugin: (i18n) Unify quoting, impove prematurally ended msgid, refs #8258.

Make sure to use double-quotes only as this is recommended i18n msgid style.

(In [10094]) TracFormsPlugin: (i18n) Extract new msgids and update German translations, refs #8258.

Minor code clean-up is do to build another msgid, that is used twice.

(In [10099]) TracFormsPlugin: Reorganisation for the main IRequestHandler, refs #8258.

Drop the 'history' action in favor of a new default form view, making use of the improved URL parser from [10096] for the '/form' base URL. And again new i18n msgids: Context-nav links show that change as well.

(In [10101]) TracFormsPlugin: (i18n) Extract new msgids and update German translations, refs #8258.

(In [10131]) TracFormsPlugin: (i18n) Update msgids and German translations, refs #8258.

(In [10143]) TracFormsPlugin: Releasing version 0.3, pushing development to 0.4, closes #3445, #3550, #3741, #4759 and #8258, refs #3388 and #6993.

This is a major release requiring a Trac environment upgrade.

While the parser logic remains unchanged, there is a lot new supplementary funcionality to make TracForms behave more like the existing Trac core resources (ticket, wiki, attachment, ...).

This version performs a series of non-trivial db schema changes, that especially may leave traces of stale forms (i.e. recorded for wiki pages, that don't exist anymore). So please make sure to read the changelog, BACKUP your environment(s) before installing this version as usual and check the new db tables forms, forms_fields and forms_history after upgrading.