Update child widget on button action changes

[thoellt]

In the current core, a WidgetActionToolButton that is re-pointed at a different action keeps showing the action it was first opened with. Changing the action is ignored, for the lifetime of the button.

This appeared when using PluginTriggerPickerAction in the FcsLoader to provide transformation plugins in the loader (e.g., do a log transform after loading in a single (user facing) step). PluginTriggerPickerAction for transformations uses a WidgetActionToolButton to provide access to transformation plugins' settings. If multiple transformation plugins with different settings are present, only the first shown settings widget would be shown.

This is fixed by connecting the already existing actionChanged signal to delete the child widget. (ll. 79-85)

The fix surfaced a second issue; the size of the widget would only be set once on start, so deleting the child and adding a bigger one would squish the ui (I already noticed similar behavior when modifying visible UI in these widgets, instead of disabling, I did not pursue this, but this might also fix, or be useful for that case).

This behavior is fixed by return sizeHint from popups if available and notifying menu on init. (rest of changes)

Example of correct widgets for two different transformations (centering and derivative) exposed through the PluginTriggerPickerAction.

When first opening the panel for the centering transform then switching to derivative and opening the panel (note the panel must be opened in both steps, due to its lazy loading behavior). The panel does not update and derivate transform shows the centering panel.