Use context for all converters (and not only Feature2Mesh)

[ftoromanoff]

The PR #2180 propose a new approach of Context for the Style, that is used in Feature2Mesh. 2 others converters also use the initial context;

• Feature2Texture.convert()
• LabelLayer and Label

This PR propose (based on PR #2180 ) to generalize the use of Context to the other converter.

Changes:

• export the FeatureContext
• move it to Style
• use it for the others converters
• modify FeatureToolTips plugin to adapt to that change

[jailln]

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