add: support for symbolic icon

[ferdiu]

• Added symbolic icon (created from the original and modified with Inkscape)
• Added the possibility to use the symbolic icon for the system tray (the entry was added to the About tab because it is not relevant in any other tab, but it wasn't worth adding an entire new tab just for this setting)
• While I was there, I updated the Italian translation (generated with AI, Gemma 4 self-hosted, but manually double-checked entirely: saved me a few keystrokes)
• Added compiled translations to the ignored list
• The png icon has been removed and the svg one is now used directly as a compiled resource in the final executable (stripped executable went from 620KB to 487KB at no cost; not a game changer but its ~21% less)
• Added the missing string in the translations for Follow system's power profile

It might seem like a minor change, but for me—and I imagine for others too—seeing colors in that corner of the screen means "new notification".

The icon colors are the ones used by the default KDE Plasma theme (breeze). The Italian translation is following the terms used in the KDE Plasma Italian translation from system settings application.

For future reference: it might be more robust to use QIcon::fromTheme method to reference icons. This way custom themes can be able to override the icons to match the style.