[applets/batterymonitor] Fix it, pt.2: Cleanup and refactoring goes on

Finally working on a battery applet. This is the second batch of patches, mostly uncontroversial ones, but require a bit of attention. Especially, applet's main file renaming and Units spacing values math.

I'm fixing the layout and implementation of the TooTipArea (the mouse hover thing on a batteries) before dropping it completely (it never shows up ever since df2469e7) just in case if anyone would want to revive it later. If you want to test it, make sure to force set its active property to true.

Replaces: !1220 (closed) because reasons Suggestions raised in those threads are still valid, and will be addressed in future patches, namely:

• BatteryItem.qml: Refactor Column which have manually set x/y properties
• PowerManagementItem.qml: Flatten two ColumnLayouts even though they have different spacing and left margins?
• Yeet ToolTipArea, since can never become active anyway.

CC @fusionfuture @ahiemstra