Since reflection and inversion introductions, compute center only with the slides of the traslations is not correct.
* `_getCenter` -> `_getOriginalCenter`
* `_getCenterWithSlide` -> `_getTransformedCenter`
* Use pango matrices to compute the new center position since matrices from Cairo, Graphene, ..., are not available with GJS.
* Stock original center and transformed center to not compute it numerous time.
Use `cr.fillPreserve, ..., then cr.stroke` instead of `cr.copyPath, ..., cr.stroke, cr.appendPath, cr.fill`
Was there a reason for painting stroke before fill ?
Is something broken ?
* Use `cr.save` and `cr.stroke` (Restore the context but not the path).
* Use `cr.identityMatrix` to clean context after transformations.
Inverting transformations was not enough, in particular with combinations of scale and rotate (`SCALE_ANGLE`, `REFLECTION`, ...).
Now all elements can be grabbed correctly.
* `SCALE_PRESERVE` (preserve ratio)
* `SCALE` (vertical, horizontal or both)
There is a third transformation, `SCALE_ANGLE`, that is not used because it causes problems to the `elementFinder`. Probably a bug of Cairo.inStroke that do no consider scales.
* 'Shapes' -> 'Tools' ('Shapes' + 'Manipulations')
* 'TextState' -> 'TextStates'
* Rotation centers are now computed dynamically (when building Cairo or SVG paths).
* Text rotation center is now located at the beginning of the text.
* Text elements gain a 'lineIndex' attribute to retrieve the rotation center of the first line.
```
JS WARNING: [/home/.../draw.js 997]: reference to undefined property "state"
```
The state property is undefined if the element is loaded from json file.
* Do not use fileInfo.get_modification_date_time for compatibility reason.
* Inverse 'Open drawing' list order: recent first.
* Overwrite identical contents when the save operation is queried by the user.
Pointer movement is ignored when the `space` key is held.
To ensure [Screenshot Tool](https://extensions.gnome.org/extension/1112/screenshot-tool/) extension compatibility, we need a way to let the third extension select the screenshot area with pointer but without leaving drawing mode and without drawing.
As a side effect, it allows to insert straight line segments in free drawing (after releasing the key).
Fixes issue #20
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