Adding folding toggle icons in sticky scroll

[aiday-mar]

Fixes: #189482

[aeschli]

I see the following log statement in the console:

[404952:0824/142840.013853:INFO:CONSOLE(210)] "Error: Trying to add a disposable to a DisposableStore that has already been disposed of. The added object will be leaked!
    at DisposableStore.add (vscode-file://vscode-app/home/martin/workspaces/vscode/out/vs/base/common/lifecycle.js:210:34)
    at StickyScrollWidget._renderFoldingIconForLine (vscode-file://vscode-app/home/martin/workspaces/vscode/out/vs/editor/contrib/stickyScroll/browser/stickyScrollWidget.js:227:36)
    at StickyScrollWidget._renderChildNode (vscode-file://vscode-app/home/martin/workspaces/vscode/out/vs/editor/contrib/stickyScroll/browser/stickyScrollWidget.js:206:18)
    at StickyScrollWidget._renderRootNode (vscode-file://vscode-app/home/martin/workspaces/vscode/out/vs/editor/contrib/stickyScroll/browser/stickyScrollWidget.js:114:73)", source: vscode-file://vscode-app/home/martin/workspaces/vscode/out/vs/base/common/lifecycle.js (210)

[aeschli]

PR is good. Some suggestions:

• Change this._foldingIconStore.dispose(); to this._foldingIconStore.clear() to make it reusable and move it up to the beginning of setState. There I would first dispose everything before starting rendering sticky scroll ranges again. I would also move the !this._editor._getViewModel() check there (after the disposes)
• topOfEndLine and endLineNumber only need to be computed when needed (on click, if not collapsed, before toggling)
• I see a small UI shifting going on when folding/expanding
  

Note that we always show folded icons (not only on hover). There's also a setting to control more: editor.showFoldingControls

Unrelated to the PR:
this._editor.getOption(EditorOption.lineHeight) is requested multiple times while rendering