Draft: [applets/batterymonitor] Fix it, pt.3: Exchange a truly bad HACK with another less horrible one

[applets/batterymonitor] Exchange a truly bad HACK with another less horrible one

GridLayout no longer crashes was described in an old comment and nowadays is pretty safe to use. Repeater, however, still doesn't support instantiating multiple top-level items at once, which is unfortunate but can be worked around; see upstream Qt bug below.

Also:

• Redoing layout and positioning, no more setting x/y inside a row/column.
• Rows and Columns were actually refactored into RowLayouts etc.
• Replace opacity: 0.5 and 0.6 with enabled: false which in terms of PC3.Label is equivalent to 0.6 opacity.

See also: QTBUG-38098

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Finally, some rly good cleanups. Next step is to yeet the tooltip entirely with all associated properties and conditions, as normally it is not displayed at all anyway.

Blocked by: !1220 (closed)