Add BlockCounter QML component which handles NotifyBlockTip signal

[hebasto]

This PR:

1. introduces additional components sub-directory for QML objects for basic GUI elements
2. adds a prototype of QML object which displays block tip height
3. handles the NotifyBlockTip signal from the core

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From Qt docs:

QML is a declarative language that allows user interfaces to be described in terms of their visual components and how they interact and relate with one another. It is a highly readable language that was designed to enable components to be interconnected in a dynamic manner, and it allows components to be easily reused and customized within a user interface.
...
It allows developers and designers alike to create highly performant, fluidly animated and visually appealing applications.

Review notes for developers

This PR continues to exploit the technique of embedding an instance of the NodeModel class into QML code as a context property (see #11). This approach differs from alternative ones, in a way that there are no signal handlers for signals from C++ code in the QML files -- only properties bindings are used.

The benefits of such an approach are:

• QML files are easy to read, reason about, and modify. Designers could easy work on them
• JavaScript code assignments in signal handlers break properties bindings

Exposing properties of the NodeModel class to QML code justifies decision to not re-use the ClientModel class because these classes have different designs.

Review notes for designers

This PR demonstrates an idea of small QML files that describe GUI elements and their interaction in a declarative way.

You could easy and safely tinker with object properties. Just run make clean && make again to see the result of your changes.

Names of QML files in the components sub-directory are actually the names of defined QML objects. Therefore, to make your life easier, you could suggest names you are comfortable with.

Don't hesitate to request new QML files that describe your design ideas :)

[hebasto]

@jonasschnelli @Talkless Do you mind reviewing this PR at a concept level?

[hebasto]

Updated 7cc9c80 -> 5467df4 (pr31.01 -> pr31.02):

• rebased due to the conflict with Share qInstallMessageHandler between QWidget and QML code #25
• the blockTipHeight property is read-only now

[hebasto]

Rebased 5467df4 -> e7b9a09 (pr31.02 -> pr31.03) on top of the #28.

[promag]

These controls have a background property:

Label {
    property int blockHeight: 0
    background: Rectangle {
        color: "black"
    }
    color: "orange"
    padding: 16
    text: blockHeight
}

[hebasto]

Thanks! Updated.

[hebasto]

Updated e7b9a09 -> d473a47 (pr31.03 -> pr31.04):

• rebased due to the conflict with Add copyright headers to QML files #34
• addressed @promag's comment

[jarolrod]

ACK d473a47

This feature works nicely and code is clean. The main thing to talk about here is the approach taken in terms of handling the Core node instance.

Here, we have a certain layer of separation between the reference to the core node and what we do at the QML layer. This should be the approach we take going forward. It is scary to be passing around this reference around raw.

Additionally, here we are showcasing why using QML is a good approach for our project. There is great separation between our business logic and the UI logic. We can have our fellow designers live in one part of the codebase where programming is not required, while us developers focus on making things functional. To me, this showcases that we made the right decision.✨

[hebasto]

Trivially rebased d473a47 -> 3ead757 (pr31.04 -> pr31.05) on top of the recent sync with the main repo.

[promag]

Code review ACK 3ead757.

[jarolrod]

ACK 3ead757

Changes since my last review is only rebase over the weekly upstream sync