fix: set initial size of widgets

[Nerixyz]

I was trying to reproduce #5411 and found this bug:

1. Set scale to something other than 1x (ideally something with a significant difference, so you see the bug)
2. Close Chatterino
3. Open Chatterino
4. Go into the settings and change the scale back to 1x (Default)
5. Correct (now): The tabs should scale correctly
   Incorrect (before): The tabs kinda stay the same (except for the font)

We didn't save the scale of the window when adding child widgets¹. So when the scale changed back to 1x, we'd assume it was 1.0 and return early.

I still can't reproduce #5411. So if anyone has a hint, please put it in the issue. Thank You!

Footnotes

1. This only works if the parent is set and if the parent has or is a window. I added assert(dynamic_cast<BaseWindow *>(this) || dynamic_cast<BaseWidget *>(this->window())) to test this locally. Only the settings pages don't set the parent. This is fine because they will always be at 1x. ↩