popover: Make the ui file match the code file name

UI files are usually named like the code file. Stick to that
pattern.

data/squeekboard.gresources.xml

@@ -4,7 +4,7 @@
    <file compressed="true">common.css</file>
    <file compressed="true">style.css</file>
    <file compressed="true">style-Adwaita:dark.css</file>
-   <file compressed="true" preprocess="xml-stripblanks">popup.ui</file>
+   <file compressed="true" preprocess="xml-stripblanks">popover.ui</file>
    <file>icons/key-enter.svg</file>
    <file>icons/key-shift.svg</file>
    <file>icons/keyboard-mode-symbolic.svg</file>

src/popover.rs

@@ -326,7 +326,7 @@ pub fn show(
         }
     });
 
-    let builder = gtk::Builder::new_from_resource("/sm/puri/squeekboard/popup.ui");
+    let builder = gtk::Builder::new_from_resource("/sm/puri/squeekboard/popover.ui");
     let model: gio::Menu = builder.get_object("app-menu").unwrap();
 
     for (tr, l) in human_names.iter().rev() {