Add "file" type to INX UI parameters

[Rena Kunisaki]

In a .inx file, it's possible to define various UI widget types like text, boolean, color... I suggest to add "file" to this list. This would function the same as "string", except it would also add a button to select a file to fill in the text field. It might have an option to verify that the selected file exists before calling the script (you'd want this for selecting existing files, but not if your script creates a new file).

[Neon22 (neon22)]

Ideally add a new type to the inx parameters which allows selection of a file by using the systems file chooser.
Existing inx params are here:
http://wiki.inkscape.org/wiki/index.php/INX_Parameters
and param defines are here:
http://wiki.inkscape.org/wiki/index.php/INX_extension_descriptor_format

Ideally a new parameter of type filename which is a string. and invokes the system file browser with options for:
- filetypes(list of extension strings) - to filter on
- allow multiple - True/False
- initial directory - including ., .., a path (unvalidated)
Which would return a list of strings of full paths. or just force it to be single only if this is easier.

However - it is worth noting that if tk was included in the distro then we could use their filechooser instead of this extra work.

[Patrick Storz (ede123)]

I'm reviewing some old bugs as I'm in the process of updating the .inx format a bit for 1.0 [1].

This is something I'd like to add if time allows.

Can people who would like to use this feature in their own extensions comment on what exactly they'd need? The goal is to keep it as simple as possible, but obviously we need to be flexible enough to cover the intended use cases.

Some questions of the top of my head:
- do we need dedicated file/folder pickers or is a general picker that
  selects files and folders fine?
- do we need multi-selection or is a single selection sufficient?
- do we need file type filtering?

I tend not to implement existence checks, as this already defines behavior (picking an existing file) whereas I'd like to implement this in a more general way that can be used both for picking an existing file as well as for choosing a new file. If the extension requires an existing file this can be checked in the extension itself and should be most flexible and error-proof. Let me know if this is an issue.

[1] https://gitlab.com/inkscape/inkscape/issues/333

[Patrick Storz (ede123)]

Let's continue discussion regarding file-picker implementation at https://gitlab.com/inkscape/inkscape/issues/344