Truncated sidebar items' font size behaves incorrectly

[Spekular]

Tested:

• Windows, master: Truncated items display as bigger(!) than non-truncated ones.
• Windows, 1.2.2: Truncated items display at a smaller size, but the truncation is still calculated for the normal size. This leaves unused space.
• Ubuntu 20, master: Font size is the same for truncated/non-truncated items
  On master + windows, when a sidebar item's full name can't be displayed, its font size increases. In 1.2.2, the font size is reduced instead, and on a master build + linux the size is unchanged.

Screenshot of Windows master behavior:

Screenshot of Windows 1.2.2 behavior:

I propose this be fixed by drawing all items at the same font size, regardless of truncation. If we want to keep the changing font size, I think the behavior should be as on windows + 1.2.2 but with truncation based on the actual font size.

[Spekular]

Tentatively milestoning for 1.3 because this should be a simple fix (removing logic rather than adding it). I haven't investigated yet though, so if this turns out to be a Qt bug at work or something else tricky the milestone can be dropped.

[Spekular]

Fixed by #5777, thanks @DigArtRoks :)