Restore basic controls.

[zwcloud]

The render-tree based rendering system broke all existing controls; let's bring all of them back.

• [x] Window 4eb39669a8c78e8e6c9b5ccabc22210612d67dde
• [x] Box 7ab5d86bb47c3c9bdcbdfd0e158bb34c54515d75
• [x] Button 4eb39669a8c78e8e6c9b5ccabc22210612d67dde
• [x] CollapsingHeader 0b31449945b7fd3ccd03846a2e3104aa2c31ae6d
• [x] ColorField 7cf5c6e517476920a449c677acc4a91844e1c878
• [x] HoverButton 5754c644745ddc0e920e715661e99da3dce312e2
• [x] Image f0019c8068a2d1761d62bc8d0b54d580d407edff
• [x] Label 4d547a75356b94e3f7d61f63987d93d698521c7c
• [x] ListBox ebf834e7d98803eec26d4c6a872800b9637369b0
• [x] PolygonButton 59516811520ef0b9cdbb08a16b1ddfbf74807ae6
• [x] ProgressBar 3c487b3e09f29e987070490054a83f3e9d6ecac5
• [x] RadioButton 2fd495362c17a6b75339b5c9992978b8c108e060
• [x] Selectable 472f8df4e430fb0f20c824cea8fa5422d0b47ee7
• [x] Separator e607db7dedb37fe0d9d2913d59655968418d81a4
• [x] Slider ad38ac3f686c33e94389dea34998687bc5ddfa6f
• [x] Space abd562190d8c9956e7b7172f5bd3441127c80604
• [x] TextBox 05b7bd38342a9d4624dcf22ff36e8a0fd5b6d265
• [x] Toggle 29cda981fee5773c7038caf5796acdd37d97c8aa
• [x] TreeNode d7378127d85ab66f90fdc42a5d0402058c0cf8b2
• [x] Child Window c146b83c9a8a402bef91454db5d80077755f5c24

[zwcloud]

After implementing DrawingVisual, we will be able to implement composite controls like ProgressBar.

UPDATE: With VisualDrawingContext, we already have the ability to implement composite controls. DrawingVisual is left to implement for drawing static vector graphs.

[zwcloud]

All basic controls restored.