Add a setting to let the user choose the icons set

There is no easy way to guess the notification area background color. This would have been a good clue to select the best icons set when Drive is starting.
The default icons set is "light" and is OK for default thems on GNU/Linux, macOS and Windows 8+.

We will workaround by:

• Using dark icons by default on Windows 7.
• Adding a new setting to let the user choose the icon them (dark or light).