A parameter with empty name (or name consisting of whitespace only)
can cause undefined behavior and should be avoided at all cost.

Empty translatable values like "gui-text"/"gui-description", while
not encouraged, might be acceptable. However they must not be
translated as gettext would return the full metadata of the .po file
in this case.

We use the user visible GUI text as value in this case, which may or
may not be translated.

For backwards-compatibility with enum items use the untranslated
value instead, if one of the deprecated tag-names is found.

Fixes https://gitlab.com/inkscape/inkscape/issues/399

* Similar to parameter's of type "string":
  Has a text entry and stores a string preference
* Additionally offers a button to show a file chooser dialog
* The node's content is the initial (default) file path.
* Relative paths will be considered relative to the extension's .inx
  file. This allows to reference files/folders shipped with the
  extension in a portable way.
  The stored value as well as the parameter value passed to the
  script interpreter will always be absolute, though.
* The attribute "mode" controls what type(s) of paths the file
  chooser allows to select. Valid values:
   - 'file', 'files', 'folder', 'folder' (pick existing items)
   - 'file_new', 'folder_new' (create a new file/folder)
* Note that manually entered values will be passed as-is without
  checking for existence.

We were actually crashing for visible parameters without 'gui-text'
otherwise.

Also make sure we actually *do* bail out if we have parameters
missing required fields and throw/catch an exception instead of
producing broken instances.

For this take a first step towards properly tracking children
of each parameter and provide functions to look up all widgets.

Seems more consistent with "int" and "float" types and should likely
be preferred over the old "boolean"

Also rename overloaded string() functions, so it's clear what they
actually do, as it's not the same thing at all...