Add blinking capabilities to FITS Viewer and Analyze.

This adds several ways to blink--that is, compare multiple images.

Analyze changes:

• In Analyze, one can now move from one session to the next (forward or backward). Keyboard shortcuts are provided for that.
• Another set of keyboard shortcuts both advance and show the next image in the same FITS Viewer. Thus, for example, one can advance through all the captured images of the evening, showing all the captures on the FITS Viewer by repeating a keyboard shortcut.
• A useful complement to this might be adding the ability to delete bad captures, but for now that will have to wait for a rainy day.

FITS Viewer changes:

• The Open File menu command (both in the main KStars top menu, and in the FITS Viewer menu) now allows multiple files to be selected. If they are then the files are opened in individual tabs. Shift-selecting would select files from the first to the shift-clicked file. Clearly one wouldn't want to select 100 files resulting in 100 tabs, but this can be used to, e.g. compare 10 images.
• Going along with the above, keyboard shortcuts have been added to move to the next or previous FITS Viewer tab,
• Also helpful to the above is a new command to zoom in/out all tabs (not just the current one).
• There is a new Blink Directory menu command (in both menus, as above) which will open a single tab with a list of all the images below the directory selected (that is, both in that directory, and in directories below it). It initially displays the first image, but new commands work in that tab to switch to displaying the next (or previous) image file in the list. This could be used to blink hundreds of files.

A demo video for these changes is available: https://www.youtube.com/watch?v=2vCOcyHcs34