Enable support for import file chooser

[stbenjam]

Some reader classes may want to have a user select a file, instead of
presenting a text box. This adds support for the feature.

It will be used by the Moon+ reader class.

Screenshot:

[aik099]

@stbenjam , you'd better also post a link to every PR you create to the https://www.mobileread.com/forums/showthread.php?t=241206 as well, because @davidfor isn't getting notifications from the GitHub.

[Geo123abc]

thank you. I desperately need this feature and look forward to the update.

[Geo123abc]

@stbenjam ，Can you send me the updated plug by you? I can't wait for having a try. Thank you.

[aik099]

@Geo123abc , what feature are you talking about exactly?

This PR adds file dialog support for bookmark importer classes, but none of the classes, except newly developed Moon Reader support (#23) uses it.

[Geo123abc]

@aik099 , I want to use the new calibre-annotations that support Moon+ Reader, but I don't know how to get it though @stbenjam had add this function.

[aik099]

@Geo123abc , theoretically these steps should help:

1. checkout [WIP] Add Moon+ Reader support #23 pull request via gh pr checkout 23 command (see more about gh command at https://cli.github.com/)
2. run calibre-debug -s; calibre-customize -b /path/to/your/plugin/folder; calibre command in the calibre installation location, where /path/to/your/plugin/folder is where gh pr ... command have cloned the PR code (as seen on https://manual.calibre-ebook.com/creating_plugins.html)
3. now you have your plugin in place

If you'll see any Python errors (in my case these were some other reader classes), then just delete files from where the errors are coming (in that PR code) and repeat the process.

[Geo123abc]

@Geo123abc , thank you.

[aik099]

@stbenjam , I completely understand you. Please be patient. We're not in a hurry here with @davidfor being very occupied with other projects and not being able to timely review any PR coming in.

I'm glad, that you're able to implement a Moon+ Reader integration, that works for you. Please don't let it disappear and not make any other Moon+ Reader + Calibre user happy.

It will be reviewed eventually I guess.

[davidfor]

This is definitely a good idea.
My apologies for the delay.