Desktop GUI: add PostProcessingWidget

[GPMueller]

This widget would be similar to the ControlWidget. It would be used to select a file from which to read images and set the spin configuration accordingly.

Features:

• Select spin configuration archive
• Play/Pause for reading images with a certain frequency
• image switching buttons and lineedit with delta (how many images to jump)
• Image switching slider
• "Record" toggle to make periodical screenshots, so that movies can be generated

Would need a toggle in MainWindow to switch modes from normal control to post-processing.

These features would make this mode a little like playing a movie in a media player, with the possibility to make cool movies in which the system is e.g. rotated slowly.

[SpiritSuperUser]

adding a post processing widget would be a great feature 👍

[GPMueller]

This could be also done in form of a ToolBar containing a play/pause button, image switching buttons and a slider. Buttons to auto-generate screenshots etc. may also be useful.

Reading and writing of configurations is currently being improved with PR #319, so a certain degree of post-processing will be available by default anyways. Runtime-variable geometry (see #102) will help a lot, as the UI won't have to be restarted when switching between data with different geometry.