Feature: Add save-as button and shortcut

[antonkesy]

This is only a working proposal. There might be still some things to discuss and adopt

Motivation

I often have small tree changes that I want to quickly test. But every save I get the File-Picker-Dialog and then I have to double-check that I don't accidentally overwrite the wrong tree file, which takes time ...

Changes

• set the default behavior of Save-Tree-Button + Ctrl + S to save the tree into the latest loaded/saved file
• add Save-Tree-As-Button + Ctrl + Shift + S to always show File-Picker-Dialog to select save location

Things to discuss/improve

Here are some items which are not in this PR (yet) and still could be added (after approval of adding this feature)

• Save the latest location in settings
• Add a modified version of save_white.svg/... for new behavior