#2 New Feature: implement optional 48x48 and 128x128 px icons

[Jan "J.J." Kischel]

As these formats are supported, user should optionally be able to add 48x48px and 128x128px icons.

Involved are these functions:
- form itself --> for this, the icons section needs to be moved to an own tab in files section
- imgtools --> add these two new formats
- open functionality
- .exe icon extract functionality
- exporter: write files if selected
- importer: open files if exist