Fixes Auto-Focus in LPE Dialog, Filter Gallery and Extension Gallery

This MR automatically focuses the search/input field when opening the Live Path Effects (LPE), Filter Gallery, and Extension Gallery dialogs. It streamlines keyboard-driven workflows by allowing users to immediately type to filter content.

Approach

1. Base Class Enhancement

   • Made DialogBase::focus_dialog() virtual to enable overriding in subclasses.

2. Dialog-Specific Overrides

   • Added focus_dialog() overrides in dialog classes to focus search boxes.

Example (LPE Dialog):

void LivePathEffectEditor::focus_dialog() {
    DialogBase::focus_dialog();
    _lpes_popup.get_entry().grab_focus();
}

Verify focus behavior in:

• Live Path Effects: Shift + Ctrl + 7
• Extensions Gallery: Extensions > Manage Extensions

Before: Search box required manual focus.
After: Search box auto-focused on dialog open.

Resolves: #5547