Add ability to select a scanner if multiple scanners are set up with Sane

[e-alfred]

Currently only one scanner is supported by this app. Sane supports multiple scanners connected to one server. Many networked scanners are nowadays available, so it should be possible to select the appropriate scanner (after it is set up in Sane) from a drop down menu or the admin/user should be able to set a certain scanner in the settings menu.

[Biont]

@e-alfred It would make sense to retrieve some information like supported resolutions/dimensions of the scanning area via scanimage -A -d $DEVICE whenever a new scanner is selected. Then we wouldn't have to do it for every single scan. (See #5 for more info).

Also, there should probably be some UI guidance that helps with new installations/upgrades where no scanner is selected yet.

And what to do if a selected scanner becomes available? Fallback to the old "just use the first" behaviour or throw an error?