Provide reusable QML components with common widgets

[chapulina]

Widgets created with Ignition GUI often have duplicate code for interfaces to vectors, poses, colors, etc. It would be easier to construct various plugins if we offered these building blocks out of the box. It would also help users to write widgets that have the same look and feel.

Desired behavior

When I'm writing a plugin, I'd like to import Ignition GUI's resources and immediately include common components. For example:

...
import "qrc:/qml"
...
IgnPose3d {
  id: myPose
  xValue: posX
  ...
}

Alternatives considered

Keep using custom components for each plugin.

Implementation suggestion

See the implementation for IgnSpinBox, which is currently used in multiple plugins.

Additional context

Igntiion Gazebo's ComponentInspector plugin already implements a lot of these. The work should consist in porting them to ign-gui and making them as general as possible.

[zflat]

I am pretty sure that if a qmldir file (https://doc.qt.io/archives/qt-6.0/qtqml-modules-qmldir.html) was added to the include/ignition/gui/qml directory and also added to the resources.qrc the qrc path to can be added to the QML load path. Then common components can be loaded with import statements as QML modules.

However, with QML modules, the name of the folder containing the qmldir is the name of the module so it would make sense to change QML files to be located at something like include/ignition/gui/qml/ignition_gui_common and put the qmldir file into the ignition_gui_common directory.

With the right setup, this wold be possible from QML:

import ignitiion_gui_common 1.0 as CommonComponents
Item {
  CommonComponents.SpinBox{
    //...
  }
  //...
}

Basically, use qmldir files to define importable QML modules.

[zflat]

@chapulina I just opened a draft pull request with changes like the ones I described in #310 (comment)

If this looks like the right approach, more files can be added to the QML module and import statements can be updated where the module will be used.

[chapulina]

We got reusable vector, pose and color widgets. That's enough to close this ticket for now. As we develop more plugins, if we identify widgets that are being recreated multiple times, we should consider following the pattern of the new Gz widgets and create reusable components for them.