svgtexttool: Some light refactoring and impl moving text by using its origin point as anchor

Alvin Wongrequested to merge
alvinwong/krita:alvin/svgtexttool-move into master

You can now move text when in the text tool by dragging its origin point. This is different from using the DefaultTool, as this applies snapping using the origin point of the text instead of one of the 8 corners or the centre that the DefaultTool uses.

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