BreakpointWidget: Fix drawing of icon when breakpoints are disabled

Fix incorrect scaling of the gap drawn inside enabled breakpoints in
BreakpointWidget when breakpoints are globally disabled and the font
size is changed from the default.

As the font size increased the gap would become larger and its center
would "migrate" downward and rightward until the gap stopped being
visible at all.

Additionally, at small font sizes integer truncation issues could result
in the gap being offset towards the top left.

- Debug font is now used in the widget.
- Row height is now determined by the character height of the current
  font.
- A `OnDebugFontChanged()` slot has been connected to properly update
  the widget on debug font changes.

| Before | After |
| --- | --- |
| <img alt="Breakpoints widget (before)" title="Breakpoints widget (before)" src="https://github.com/user-attachments/assets/fdde293a-b1fd-4a1d-85e2-e16bc859eb9e" /> | <img alt="Breakpoints widget (after)" title="Breakpoints widget (after)" src="https://github.com/user-attachments/assets/cf4f1d59-c235-429b-bcb1-cae36065e805" /> |

Allows you to quickly stop breaking, play the game, then re-enable breaking. useful if you have many active breakpoints, but need to run the game.

DolphinQt: Replace widespread SetQWidgetWindowDecorations calls with an event filter.