PixelPaint: Add Color Layers

[TobyAsE]

This PR massages a lot of code regarding Layers... But in essence a new Layer-Type is born: solid-colored, non-tool-editable Layers.

When creating a new Layer you can choose the Type: Default Bitmap or Color:

When choosing Color you then can select the color the new Layer will have:

Masking works also on the color-layers, and the Layer properties know about the Color if applicable:

[BuggieBot]

Hello!

One or more of the commit messages in this PR do not match the SerenityOS code submission policy, please check the lint_commits CI job for more details on which commits were flagged and why. Please do not close this PR and open another, instead modify your commit message(s) with git commit --amend and force push those changes to update this PR.

[djwisdom]

More love for PixelPaint, add color layers it is. ❤️

[awesomekling]

Out of curiosity, what does one use a solid color layer for?

[stale[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions!

[stale[bot]]

This pull request has been closed because it has not had recent activity. Feel free to re-open if you wish to still contribute these changes. Thank you for your contributions!