Importing an icon set should remove unused icons of previous icon set

[hofstef]

Describe the bug
[A clear and concise description of what the bug is.]

To Reproduce
Steps to reproduce the behavior:

1. reset the icon configuration to default
2. import the attached icon set which differs from the default icon set: 'document' was removed, 'alarm' was added
3. 'document' and 'alarm' are selected as work objects

Test icon set (rename to test.iconset):
test.iconset.txt

Expected behavior

• 'document' was removed from palette
• 'alarm' was added to pallete

Version

• Egon: v2.0.1

[hofstef]

Released with v2.1.0