WindowBase: Update bind list of properties: system_size, size, width, height and center

[pythonic64]

Changelog:

• Removes softinput_mode and keyboard_height from bind lists to address 2284c2a#r33486046 comment
• Adds _density to bind lists of width and height and removes it from center as center depends on width/height which makes dependency on _density implied
• Not caching center because of width and height won't trigger caching in all cases.

Property center and caching problem: #6433

Maintainer merge checklist:

• Title is descriptive/clear for inclusion in release notes.
• Applied a Component: xxx label.
• Applied the api-deprecation or api-break label.
• Applied the release-highlight label to be highlighted in release notes.
• Added to the milestone version it was merged into.
• Unittests are included in PR.
• Properly documented, including versionadded, versionchanged as needed.