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.

This is easy to miss and causes confusing behavior
(e.g. wrong page/option selected)

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

Renders a Gtk::TextView that automatically fills the available
space in the extension's preferences dialog and can be used
to accept longer multi-line strings.

Newlines in the string value will be passed as "\\n" to the
extension script (i.e. literal '\n' with the backspace escaped).

Renders a simple Gtk::ColorButton with label instead of the full
ColorNotebook which is too large for many applications.

Should likely be re-implemented bringing up a new Dialog that wraps
an Inkscape::UI::Widget::ColorNotebook eventually, instead of
relying on the Gtk::ColorButton's picker implementation which is
inconsistent to existing pickers.

* 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.

* The node's content is the file path.
  Absolute paths should work, the preferred way is to specify a
  relative path, though, which will be interpreted relatively to the
  .inx file's location
* The attributes "width/height" allows to override the native size
  of the image.