Implementation of the click-selection operator of keyframes in the dopesheet, along with its alternatives : deselect all when no frames is hit, extend selection with shift, column selection with alt, select all keyframes on current channel with ctrl+alt.
Includes the new following API functions :
* `select_frame_at` : selects a frame in a layer at a specific time (if such frame exists),
* `select_all_frames` : selects all frames of a layer,
* `select_layer_channel` : selects a layer, and sets it as active (if layer is not null, otherwise the active layer is set null),
* `layer_has_frame_selected` : checks if any of the frames in the layer is selected.
Pull Request: https://projects.blender.org/blender/blender/pulls/110492
0c07fb50c8