* Create a custom PixeloramaImage class, initial support for indexed mode
* Convert opened projects and images to indexed mode
* Use shaders for RGB to Indexed conversion and vice versa
* Add `is_indexed` variable in PixeloramaImage
* Basic undo/redo support for indexed mode when drawing
* Make image effects respect indexed mode
* Move code from image effects to ShaderImageEffect instead
* Bucket tool works with indexed mode
* Move and selection tools works with indexed mode
* Brushes respect indexed mode
* Add color_mode variable and some helper methods in Project
Replace hard-coded cases of Image.FORMAT_RGBA8 with `Project.get_image_format()` just in case we want to add more formats in the future
* Add a helper new_empty_image() method to Project
* Set new images to indexed if the project is indexed
* Change color modes from the Image menu
* Fix open image to replace cel
* Load/save indices in pxo files
* Merging layers works with indexed mode
* Layer effects respect indexed mode
* Add an `other_image` parameter to `PixeloramaImage.add_data_to_dictionary()`
* Scale image works with indexed mode
* Resizing works with indexed mode
* Fix non-shader rotation not working with indexed mode
* Minor refactor of PixeloramaImage's set_pixelv_custom()
* Make the text tool work with indexed mode
* Remove print from PixeloramaImage
* Rename "PixeloramaImage" to "ImageExtended"
* Add docstrings in ImageExtended
* Set color mode from the create new image dialog
* Update Translations.pot
* Show the color mode in the project properties dialog
