Support multi-image mode to remove the need to invoke the plugin for every single mage

[capricorn8]

At least a basic way of moving back and forth in a list of images selected in Lightroom should be supported, as this would significantly ease (mass) testing on Nikon images by users for V2.2.

[capricorn8]

In a first step, the multi-image mode will be an automated single-image operation. The user can move fowards and backwards in a series of photos that have been selected in Lightroom before running the plugin:

The "Calculating focus point" dialog will still be displayed and a new window will be created for the photo's focus point display.

Dialogs with error messages that have previously stopped the plugin's execution (e.g. "No focus information found") have been removed as far as possible with reasonable effort. The corresponding messages will be displayed in the focus point viewer windows instead:

For the future it is intended to not have changing windows and separate dialogs but one permanent window with fixed dimensions that is handling the photo and info display, as well as status and error messages.