feat(qt): search boxes clear on Escape, drop focus, and offer a clear cross

Every search field follows one contract: Escape empties the text AND hands
the keyboard back to the shell root, and a clickable cross clears the text
while keeping the caret in the field. The header search shows the cross
left of the Enter glyph while the cortinilla is open; QbzLineEdit's search
mode and the new QbzSearchField (the album track filter and the playlist
filter) float it at the right edge.

Each field also keeps its focus after an edit unless another text input
took it, and logs when it had to reclaim it.

cc

crates/qbz-qt/build.rs

@@ -597,6 +597,7 @@ fn main() {
                 "qml/controls/ScrollMemory.qml",
                 "qml/controls/ScopePanel.qml",
                 "qml/controls/QbzSegToggle.qml",
+                "qml/controls/QbzSearchField.qml",
                 "qml/controls/QbzSelect.qml",
                 "qml/controls/QbzSelectableText.qml",
                 "qml/controls/QbzSplitButton.qml",