Add intermediate points to make quick free drawings smoother.
Do not redisplay the area at this step (crashes).
Use device.get_coords rather than global.get_pointer because the later return rounded (floor) values.
Fix#45.
* Import palettes from a .gpl file (one palette for each column).
* Add possibility to use more than 9 palettes.
* Color display names support.
* color.toJSON for svg and drawing saves, color.toString for display.
The color picker use the ["Color Picker" extensions](https://github.com/tuberry/color-picker) if it is available thanks to `pickAsync` "api". In comparaison to the GS color picker, it provides a colored cursor and a menu to adjust the color.
* element.x and element.y are relative to the area, not to the stage.
* Do not hide and do not set opacity to 0 because ibusCandidatePopup need a mapped text entry to init correctly its position.
* Rename "Persistent" to "Persistent over restarts".
* If disabled, "Persistent over toggles" has priority over "Persistent over restarts" and "Drawing on the desktop".
Close issue #42.
* Pass Clutter color and Pango fontDescription objects to elements, not strings.
* Do not use the color format '#rrggbbaa' in svg since it is not supported in GNOME 3.24.
* 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.
* It is no longer required to clamp the setting value in area.js.
* Add a "range" property, completed with another 'step' property, to PixelSpinButton in order to directly initiate the lower and upper values of the adjustment from the relevant setting range.
* Not related: fix an inversion between Pango and Cairo in element.js.
Since the most drawing params can be changed via menu or shortcuts, remove default.css and user.css.
* Missing params like palettes, dash, grid, etc, are added to prefs.
* Add palette choice to menu and shortcuts.
* Some defaults like fonts, line width, etc, are no longer modifiable by the user.