[forms] Allow users to copy and paste form widget configurations from one field to another

[gacarrillor]

When configuring forms (e.g., using the Drag and Drop Designer) in layers with multiple fields, chances are we want to set similar or even the same configurations for a number of them. This process is currently manual, which ends up being a bit tedious and error prone.

This PR adds a handy context menu (in the Vector properties dialog, Attributes Form tab, Available Widgets tree, Fields category, :) ), allowing users to copy widget configurations between fields, and thus, saving time and making sure of not missing a particular setting by mistake.

Implementation details:

• Copy&paste of widget configuration can be done between fields of the same layer, between fields from different layers in the QGIS project, or between fields from layers in different QGIS instances.
• Pasted widget configuration overwrites the target one, but is not applied automatically, so that users can see the pasted configuration and possibly tune it and apply it or avoid saving it by cancelling the dialog or just selecting another field.
• We avoid copying aliases and comments. The target field ones are respected.
• A widget configuration can be copied and pasted on the very same field, thus adding support for keeping a widget config in the clipboard, making and testing changes and eventually reverting them by pasting the clipboard content in the same field.
• We only paste "editor widget configs" when the target field supports the source editor widget type. Otherwise, we leave the target editor widget type and config intact, while showing a warning message (in dialog's message bar) explaining why the config could not be pasted.
• Target provider constraints are kept, we only paste constraints with layer origin.

Fix 58971

Funded by (?)

[gacarrillor]

@3nids, before showing this to the world, could you please have a look at this PR?

Do we need QGIS_PROTECT_QOBJECT_THREAD_ACCESS in both copy and paste slots?

Note: I'll still tweak pasting constraints, since the use case of pasting an unset constraint on an already set one is currently missing. Done.

[github-actions]

🪟 Windows builds

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Debug symbols for this build are available here.
(Built from commit 0a63c26)

🪟 Windows Qt6 builds

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(Built from commit 0a63c26)

[3nids]

Looking good!

[gacarrillor]

@3nids, do you know if we should add \since QGIS 3.42 to methods that are public with respect to their class but are not part of the public API (i.e., marked as SIP_NO_FILE)?

[3nids]

no only to the class