Add static blur for dash-to-dock

[aunetx]

Now that Dash to Dock seems stable for gnome 40/41, I can provide an option to use a static blur -- just like for the panel.

This would be cool because:

• no more artefacts
• the dash content should be more readable (due to the background being less uniform with the window behind)
• rounded corners!!!

I will however add a switch (just like for the panel), because:

• no more dynamic blur, which was kind of the point of the extension at the beginning
• we're an extension, so I don't have any preferences guideline! :p

The switch could be switched on by default, because it provides a more polished user experience :)

[Arcitec]

Hi aunetx, thanks for everything. This idea sounds great.

I also want to make everyone aware that Dash to Dock already has a method to make it look great:

Under its Apperance tab, set to Dynamic:

You can click the "cogwheel" to customize the minimum and maximum transparency.

It will then behave as follows:

• When there are no windows behind the dock, it will be very transparent and blend with the wallpaper.
• When there are windows it will be slightly transparent and will slightly blend with the app under it.
• The dock uses rounded corners.

Changing the transparency amount (via the cogwheel) lets people customize this.

I am happy with Dash to Dock's own method, but I am curious what your idea will look like! :)