Improve Themes window
Some improvements for theme handling in Gajim's themes window.
- Adding a new theme will now automatically apply the new theme. This enables users to view their changes directly without switching the active theme in Preferences first.
- Deleting the currently active theme will fallback to default.
- Buttons were rearranged a bit and tooltips were added.
- a placeholder text is shown if the selected theme has no options yet
- I propose to keep the 'Add Setting' button always sensitive. This enabled the user to directly add a setting without having to add a theme first. A new theme is added automatically if there are no themes.
Fixes #9886 (closed)