#38 Bring back "Extract page" from Comix 3

[Oddegamra]

I've been missing this particular feature from Comix 3 which allowed one to directly save the currently opened page to an arbitrary location (especially for sparing one the trouble of manually opening the archive, finding the right page, saving, and so forth).

This patch re-integrates such functionality in the popup menu and file menu. Line numbers might be slighly off (I have some other modifications in my tree), but the rest should apply fine.

[Oddegamra]

Bring back "Extract page" from Comix 3

[Half a Page]

This patch works great!
But it would be very nice to have it reintegrated back. I agree that the rightclick menu should not be cluttered, so I propose adding it to the top menu somewhere. The feature was very handy.

[HoverHell]

Thanks. Quite a useful feature sometimes indeed.
Ihave applied it into my fork repository (github.com/HoverHell/Comix/).
Though, there's couple more things to do (maybe I will get to do that myself):
1. Using the last `extract file` directory for the file choosing dialog (instead of the current directory).
2. If file is not from the archive, prepend its directory name (just as archive name) to it. Maybe.