Themes window: Add 'Apply this theme' button

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Versions

• OS: Windows 10 x64
• Gajim version: Night Build 07/Nov/2019

Steps to reproduce the problem

1. Try to change font size from 10 to say, 14. Press select.
2. Try to type in chat and see fonts haven't changed.
3. Go back to font selector and see that nothing has actually changed.

Expected behavior

Font size should change when changed.

Actual behavior

Font size stays the same even when you try to change it.