Familiar shortcut commands

To maintain familiarity with industry software, here are some commands that we should aim to develop:

- AltDelete/Backspace: If one layer is selected, fill it completely with the primary working color
- AltShiftDelete/Backspace: If one layer is selected, fill its color channels (preserving transparency) with the primary working color
- CtrlDelete/Backspace: If one layer is selected, fill it with the secondary working color
- CtrlShiftDelete/Backspace: If one layer is selected, fill its color channels (preserving transparency) with the secondary working color
- ShiftDelete/Backspace: Open a menu to choose the settings for a fill node that can optionally preserve transparency, choose a blend mode and opacity, and pick from what custom paint type (color/gradient/pattern) to use

This list will expand over time.

The issue names no files, tests, or entry points. Start by locating the application's keyboard shortcut handling and fill-node behavior, then compare the implemented commands with the five listed shortcut groups; done means the specified fills, transparency behavior, colors, and fill settings are available through those shortcuts.

Written by the indexing model from the issue text.