compress-dialog: Use GtkDropDown for format selection
The system-lock-screen-symbolic
icon is misplaced in the AdwActionRow
widget after the GTK 4 port and there is no easy way to fix this. Let's
port the whole GtkPopover
for format selection to GtkDropDown
instead.
This also allows adopting the originally intended design.
Related: #822 (closed) Fixes: #2084 (closed)