a11y(connections): tree rows and form fields lack semantics; some actions only work with the mouse #1822

Problem

Found by driving a debug build through its semantics tree (dev at 25c766c):

• Connection tree. The querya database row of a MySQL connection exposes a button with no label, and tapping it through the semantics action does not expand it; only a real mouse click does. Keyboard and screen-reader users cannot expand the tree.
• Connection forms (MySQL, MongoDB). The Host, Port and Username fields have no label in the semantics tree (only the "Host" / "Port" captions next to them, as separate text nodes). The password field has two nodes, Show and Show password, for one button. The Environment buttons are labelled twice ("None\nNone", "Development\nDevelopment").
• Select your database. Four of the five tiles (PostgreSQL, MySQL, Redis, MongoDB) are image nodes, only SQLite is tappable.

Expected

Every tree row is a labelled node with expand / collapse and select actions; form fields carry their caption as the label; one node per button.

Where

querya_connection_tree_row.dart, the shared connection form sections (connection_host_port_section.dart and friends), new_connection_dialog.dart.