* Let the user choose the directory.
* Rework of Files.Images.
* area.currentImage is now an ... image, not an index.
* Add 'switch-image-file-reverse' keybinding.
* Add image gicons (from thumbnails) in the submenu.
* Images are reset on drawing mode left or on images added from the clipboard.
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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