qml: Add DropShadow to the OptionSwitch indicatorButton

[shaavan]

Ported #111 to this PR.

This PR rebase #111 on the current master.

Notes:

• I successfully compiled the updated PR on Ubuntu 22.04 using Qt version 5.15.3.
• To successfully compile and run the PR, I had to install an additional library.

sudo apt install qml-module-qtgraphicaleffects

• I have created a convenient demo for checking that the drop shadow works correctly. Drop Shadow onboarding test branch cherry-picks this PR over the Onboarding views #124.
• If, in the demo, the color of the drop shadow is too subtle to perceive, try experimenting with changing the color of the drop shadow to get high contrast. For example:

             spread: 0.0
             samples: 21
-            color: "#00000040"
+            color: "#7900fffc"
             source: indicatorButton
         }
     }

[jarolrod]

CI is failing because you have to allow static builds to pull in these plugins, here's an example on how to accomplish this: https://github.com/bitcoin-core/gui-qml/pull/45/files

[hebasto]

@shaavan

Could you post screenshots please?

Also I'd like to have some Concept ACKs from designers before introducing a new plugin.

[hebasto]

Why importing of private plugin is required?

[shaavan]

In the recent Bitcoin Core App Design Call on 06-29-2022, it was discussed the need to introduce a dependency qml-module-qtgraphicaleffects for the DropShadow is not worthwhile. And hence, adding DropShadow should be postponed until the same dependency is used for some other important task.

So I shall be closing this PR until later.