event-widget: Style overhaul

This reworks the style of the event widgets to make them match the latest designs. This also supports the light and dark theme preferences and fixes some spacing inconsistencies and errors.

• The orientation of events only affects how their content is laid out, not their style.
• Events are fully colored accordingly to their calendar's color.
• Timed events have a semi-transparent background and a kind of opaque knob on their left.
• Timed events have their text colored according to the theme.
• Lasting events are fully opaque, with no knob.
• Lasting events have their text colored according to their calendar's color.

This also uses the Libadwaita style loading mechanism.

Ping @snwh for design review.