More Plasma Scripting Cleanup

This is another round of cleanup of the plasma scripting docs.

• Update Containment properties
• Containment.screen is readonly
• Containment.name doesn't exist
• Containment.desktop doesn't exist (virtual desktops are controlled by KWin Scripts)
• Document Containment.addWidget(string name, number x, y, w, h)
• Ignore documenting Containment.addWidget(string name, number x, y, w, h, array args) since there is zero examples.
• Move common properties and funcs to Applet section
• Fix wrong Activity references which should be Desktop.
• Use consistent Java generic syntax array<string> instead of using square brackets in half the page array[string].
• Mention Applet.type and Applet.version reads from the metadata.
• List inherited functions without a description unless necessary. The inherited class is mentioned for easy lookup below.
• Specify Applet.id is a numerical instance id.
• Add inherited functions/properties.
• Mention functions are global functions and not object methods.
• Removed the (scriptingVersion >= 2) since scriptingVersion=20 hasn't changed since (at least) 2014. The most recent change didn't bump it either.
• Indent lists to use a consistent indent.
• Link QKeySequence
• Document userBackgroundHints
• Replace indexOf() example with includes()
• Fix new array() example where Array should be capitalized
• Document that knownWidgetTypes lists all plasmoid types including containments. I need to also mention that knownPanelTypes and knownActivityTypes return empty lists but it might be better to leave them undocumented for now.
• Mention Containment.locked. Should probably mention it's deprecated?
• Fix // typo in link, gitlab doesn't like it.
• Linkify Text in QML ColumnLayout example