Allow overriding Text and Image type to custom user type

Some customers would like to replace the Text items with custom ones.

Suggestion:

Allow a global option to replace Item and Text with custom types.

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In Advanced settings we can have Layers - QML Item mapping.

i.e

Asset layers default mapping would be 'Image' item and Text layer's default mapping would be 'Text' item.

These mapping can be changed and this would be saved as the document settings.