applets/notes: Workaround initialization bug (+minor code improvements)

list of changes, in the order from boring to interesting, so better read from the bottom:

Set more efficient background for TextArea

Clean up unnecessary id

Explicitly specify arguments in signal handlers

Injection of parameters into signal handlers is deprecated. Use JavaScript functions with formal parameters instead.

Fix check on attached object Window

Attached objects always exist whenever you query, set or bind them or their properties. This particular line of code meant to check if item is presented in a window, to guard next subexpression that reads flags.

Move DocumentHandler inside full representation

PlasmoidItem's setting preloadFullRepresentation: true causes a glitch of initialization order: full representation component is created immediately when the toggle is flipped ON, which is practically right in the middle on QML Engine being busy creating PlasmoidItem as a whole and populating its properties and bindings. Essentially there is a risk of observing something normally unobservable: QML objects' id references may be null on read! So the signal handler onCfgFontPointSizeChanged in TextArea was failing on start with an error

main.qml:214: TypeError: Value is null and could not be converted to an object

which can be interpreted as "I know that documentHandler is an object property, it exists and I can try to read it, but currently its value is null". That definitely is not something that's supposed to ever happen in QML land, but something that can be easily patched up in applet. Besides, there is no reason why DocumentHandler should exist outside of full representation component, and relocation simplifies its bindings by getting rid of conditionals (which theoretically were not needed anyway due to preloading in the first place).