flip|cover-switchers: Improve transparent and blurred background

Merged Ismael Asensio requested to merge work/iasensio/switchers-background into master

Currently, the flip|cover window switchers use a Plasma Dialog as their background, which can provide transparency and blur but it's totally dependent on the plasma style, which may be not be designed for this purpose, and doesn't look clean with current breeze defaults.

Use instead the same components that are used in the Overview effect, for consistency and simplicity. It should also help with efficiency, as the background and blur effects are done over DesktopBackground special component and not over the actual desktop.

The code is also prepared with a bool property to not blur the background, but it doesn't add the user configuration part (yet).

There are some BR comments asking for this, but more on a forum style with multiple requests/rants, so I'd rather not link the BR here.

Some examples with Breeze Plasma style, and light and dark colorschemes

Light Colorscheme Dark Colorscheme
Current coverswitch-breezelight coverswitch-breezedark
After coverswitch-breezelight-new coverswitch-breezedark-new
No Blur (possible future option) coverswitch-breezelight-noblur coverswitch-breezedark-noblur

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