st/widget: Emit "style-changed" on all theme changes

st/widget: Emit "style-changed" on all theme changes

Since now the resource consumption of the "style-changed" signal
handlers should be significantly lower, we can start emitting the signal
in all cases again as it was originally intended. This fixes some small
visual issues, like the battery percentage in the panel only being shown
on hover or updating the size of StIcons on scale-factor changes.

This effectively reverts f74c07b9acbdc6945ca37f86bc680701f048f174

Fixes https://gitlab.gnome.org/GNOME/gnome-shell/issues/1708

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2099>