RFE: "Save preferences after exit" and "clear all set" button

[belonesox]

Problem: every time than user run the app, the user (highly likely) have to customize size/border color/thinkness. And then she have to exit and rerun the app for every collage, because there is no fast way to clear image set.

Possible solutions:

• make "clear all set" button
• save last used preferences
• make preferences somehow loaded with customized templates (now they are hardcoded).

I fix my regular setting in code, but this solution is not suitable for regular user.

[ChrisLauinger77]

+1 for the clear button
+1 for the last used preferences to be saved
+1 for the customizable templates

[ChrisLauinger77]

I made a PR for save last used prefs
and to have a json file for templates rather then have them in the code.
Saving new templates is not implemented - for now the JSON can be edited by hand to add more.
see #113

[ChrisLauinger77]

Added also the reset button to #113