WebUI improvements

[Chocobo1]

• WebUI: remove child elements directly
• WebUI: use correct property for selecting child elements
  firstChild will select the first Node which is often not intended (it should be Element instead).
• WebUI: use idiomatic string methods
• WebUI: use proper event for handling text changes
• WebUI: fix wrong event property
• WebUI: fix invalid style
  initial isn't applicable to borderLeft/borderRight.
• WebUI: fix checkbox initialization
  Previously the checkbox had all options checked regardless of the stored setting.
• WebUI: revise Edit Category dialog button text
  The generic "OK" is suitable for more scenarios.
• WebUI: locate element faster

[thalieht]

Since i have no idea what almost all of these changes do i started testing random stuff.

Caused by this PR:
Filters side panel: tags and trackers text are missing, both light and dark theme.

Not caused by this PR:
Remove torrent dialog: Cancel has no effect and remove doesn't close the dialog
Rename files dialog: Close button has no effect
Edit category dialog: Button is called "Add", add what? Shouldn't it be "Save" or "Ok" like in GUI?

[Chocobo1]

Since i have no idea what almost all of these changes do i started testing random stuff.

Appreciated the testing!

Filters side panel: tags and trackers text are missing, both light and dark theme.

My fault, fixed.

Remove torrent dialog: Cancel has no effect and remove doesn't close the dialog
Rename files dialog: Close button has no effect

I was looking into it too earlier. Those are regression from #21804 and #21892. I've already fixed them in my unpublished branch. I'll submit a PR later. Fixed in PR #21919.

Edit category dialog: Button is called "Add", add what? Shouldn't it be "Save" or "Ok" like in GUI?

I've changed it to "OK".