UX of ExifTool options in Hugin Preferences

[Milan Knizek]

While I encountered the bug reported here https://bugs.launchpad.net/hugin/+bug/1685928, I have spent some time trying to understand what is expected from the user in Hugin / Preference / Stitching (2).

"Choosing" an arg file was not the obvious choice, since I have not create any yet.
So I started reading the documentation of ExifTool and searching the web for some examples. Then I tried to look for specific hugin's user guide / wiki on how to define variables in the file. I did not find any.
(The syntax should be like "-Model=some text", but how to make the use of variables when the text in the prefs reads "Placeholders will not be replaced" and I even do not know what the placeholders are?)

So, "editing" the arg file looked like a way to go - I filled in an arg file name in to the field ("test") and clicked "Edit". Now, it became obvious what the template looks like ("Model=", etc.)

Unfortunately, I did not enter the /absolute/path/name/to/test and as a result, the file disappeared the next day. The culprit was that my temporary directory is /tmp and the "test" file was created right there in /tmp/test... And my /tmp gets wiped every reboot.

P.S. My journey was even a bit more miserable, because of the bug reported above - no matter what, no EXIF was copied to my output panorama. So I thought the errror had been on my side due to wrong syntax in arg file. Having looked at Hugin's source code I found that "Edit" can also create the file. Call me stupid, but this really took me quite lot of time...

My proposal is to:

* add the text "argfile may be left empty for the default settings";
* add the text "argfile name must be an absolute path name" (or changing the code to place the file in some directory, like $HOME/.hugindata folder);
* the "Edit" button should change its description to "Create / Edit". (Edit is for _existing_ file and as such provides no hint that it can also create the file.)

[tmodes (tmodes)]

> * add the text "argfile may be left empty for the default settings";
The tooltip says already "leave empty for default". But that seems to complicated.

About the other things I have to think.

PS: Concerning the exiftool syntax. It is not so easy. Exiftool is very flexible. But this high flexibility requires a lot of different switches. So the "-Model=Text" syntax is only one kind of assignment. For list like tags (like Keywords) the syntax is different. So writing a GUI to hide this complexity would be an enormous task, and beside the scope of Hugin.

[tmodes (tmodes)]

I committed some changes to default repository. (Not all was implemented, e.g. the button title remains at edit. The longer description can result in even longer text when translating - then the button is growing extreme. Instead a longer description is added above.)