fix: keep drun grid columns and icon slots stable

[zouyonghe]

Problem

With drun icons enabled, GTK FlowBox laid out entries using natural child widths. During asynchronous icon loading, the grid could change from four columns to three, and different icon aspect ratios could shift neighboring entries.

Fix

• Keep the inner application FlowBox homogeneous.
• Set both minimum and maximum children per line from the configured column count.
• Use a horizontal inner FlowBox when the outer layout is vertical, keeping the search box above the grid.
• Constrain displayed labels and reserve a fixed image_size slot for each GTK image widget.
• Preserve the original action/search text; only visual labels are ellipsized.

Verification

Tested on Hyprland with Wofi drun, icons enabled, four columns, and a 3840x2160 display. Early and settled screenshots keep four columns and fixed icon slots; no GTK warnings were emitted.