fix: Force refresh of theme and stylesheets when swatch color changes

[jperedadnr]

Issue

Fixes #764

When the swatch color property changes, the list of external stylesheets applied to the current theme changes too, so the editorController needs to make another call to retrieve those, if any (for instance, when the Gluon plugin is present), and refresh content/preview/... accordingly

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[abhinayagarwal]

why is setting the theme to null required?

[jperedadnr]

EditorController has themeProperty with a default theme. When the theme changes, it refresh the scene graph from the invalidated() method, and there are a bunch of listeners in different places (5 so far) attached to themeProperty() that react accordingly.

For instance, in ContentPaneController we have:

editorController.themeProperty().addListener((ov, t, t1) -> themeDidChange());

that ultimately calls EditorPlatform::getStylesheetsForTheme, that brings in the Gluon plugin stylesheets and the updated swatch color stylesheet.

The easy way to trigger all those listeners is to force an invalidation of editorController.themeProperty(), and that's easily done with setting null and restoring the current theme.