Fix alignment of icons in Places panel and Compact view mode

Summary:
Adjust calculation of icon pixmap Y value; now based off center of text label bounding rectangle. Previously, icons appeared top-aligned when text size was larger than icon size.

Centering is done by obtaining the center point of the text frame (`m_textRect.center().y()`) and setting the top `Y` point of the icon to one-half of the scaled icon height (`m_scaled_PixmapSize.height()`)  Division is done by `2.0`, to ensure calculations are done with floating-point math, in keeping with `QPointF`, which returns floating-point values.

Also includes an adjustment named `midlineShift` (contributed by @zzag), which takes into account the font's midline in respect to the center of the text frame. Certain fonts (i.e. Noto Sans) can have a slightly offset midline, resulting in imperfect alignment of the icon. This small adjustment resolves that potential issue.

See before and after screenshots.
{F5764918}
Before, showing misalignment (with and without debugging wireframes)

{F5764920}
After, showing correction

BUG: 390771

Test Plan:
-- Compile Dolphin with patch
-- Increase system font size by several points (i.e. 15pt)
-- Check that Places panel icons remain centered with the text label
-- Select Compact View mode
-- Adjust icon size slider to minimum
-- Ensure that icons also remain centered in file listing
-- Check several combinations of icon size & font size to ensure centering is consistent

Reviewers: #dolphin, ngraham, cfeck, elvisangelaccio

Reviewed By: #dolphin, ngraham, elvisangelaccio

Subscribers: zzag, elvisangelaccio, #dolphin

Differential Revision: https://phabricator.kde.org/D11650