Fix hardcoded app icon

[HEXcube]

As u can see from pomodoro-indicator's desktop file, app icon is wrongly placed in /opt/extras.ubuntu.com/pomodoro-indicator/bin/pomodoro-indicator and Icon= field set to a hardcoded path. 😟

According to freedesktop icon naming specification (the standard for Linux desktop), the correct way to declare icon in desktop file is:

Icon=pomodoro-indicator

For this change to work, pomodoro-indicator.svg icon has to be installed in the standard icon location:

/usr/share/icons/hicolor/scalable/apps/pomodoro-indicator.svg

Fixing this has the additional benefit of adding icon theme compatibility to Pomodoro Indicator's desktop entry. ☺️ See: What are hardcoded icons? and Foggalong/hardcode-fixer#221