Add keep-color prop to QTree

[duffyg]

What kind of change does this PR introduce?

• Bugfix
• Feature
• Documentation
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes
• No

The PR fulfills these requirements:

• It's submitted to the dev branch (or v[X] branch)
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• It's been tested on a Cordova (iOS, Android) app
• It's been tested on an Electron app
• Any necessary documentation has been added or updated in the docs or explained in the PR's description.

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to start a new feature discussion first and wait for approval before working on it)

Other information:
The checkboxes in the QTree component do not keep color when using the default color. See here: https://quasar.dev/vue-components/tree#example--syncing-node-properties (the "Pleasing decor" leaf is unticked and has a grey color)

However, when selecting a different color using the control-color prop, then keep-color is automatically applied.

This is inconsistent. Also, users may wish to switch off the keep-color prop the same way they can for the QCheckbox component.

This PR adds a new keep-color Boolean prop to Qtree, defaulting to true to avoid affecting existing QTree instances. This new prop is applied to the keep-color prop of all checkboxes in the QTree.