Add insert query placeholder button to search provider dialog

Add a 'Insert query placeholder' button to the add/modify web shortcut
dialog. Clicking this button inserts \{@} in the shortcut URL lineEdit.

This makes it easy to add new web shortcuts.
Users don't have to remember and type the query placeholder.
BUG: 146879
FIXED-IN: 4.11
REVIEW: 110423
GUI: Button added to the add web shortcuts dialog.