Add the Structure tool window

[Leitwolf11]

The PSI module has returned a declaration outline since #18, and nothing in the editor showed it. This adds the Structure tool window.

What it does

• Lists the declarations of the active Visual X#, Kotlin, Java or Groovy document as an indented list, from the outline of SourceAnalyzer.
• Selecting a declaration moves the caret to its start. The declaration around the caret is marked.
• Docked on the left below the Project tool window; the two share the column and divide its height when both are open. It opens from the stripe or with Alt+7 and is closed when Xide starts.

How it stays correct

• The outline is read on a background thread, 200 ms after an edit, and only while the tool window is open.
• The rows carry the document version they were read from. A row of an older text does not navigate, because its offsets would address other source; the list is replaced as soon as the new text is read.
• Python has a tokenizer and no parser, so the window says that it has no structure. A text longer than the colouring limit is not parsed.

Tests

• StructureTest: rows from an outline, the innermost declaration around an offset, version check, the structure of each language, a text that does not parse, the empty cases.
• StructureUiTest drives the real Compose shell: listing, hiding, navigation and the marked row, an edit, Alt+7, Kotlin, Python and no document.
• ./gradlew check passes locally on Windows.

Not verified

• The window was exercised through the semantics tree only. Nobody has looked at it on screen yet: the glyph on the stripe, the one-letter kind marks and their colours need a look.

Not in this change

• Resizing the left column, a tree with collapsible nodes, sorting and filtering.
• PSI syntax errors of Kotlin, Java and Groovy in the Problems tool window.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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