GridView: Create dummy grids only when no apps to populate

[ryonakano]

Refactoring as a GTK 4 prep. As requested in #710 (comment)

Current approach in master

1. add_new_grid (): Create dummy grids for all seats
2. populate (): Overwrite it with AppButton

Problem

The above 2. won't work as expected in GTK 4 as I wrote in #710:

• get_widget_at() called in focused_column and focused_row properties returns Gtk.Grid instances instead of AppButton ones, resulting keyboard navigation does not switch focused app entries

New approach in this PR

1. populate_page (): Create dummy grids only when no more apps

Note: link_args: ['-lm'], is added to the meson.build to use Math.ceil () as written in the valadoc

Checklists

• Confirmed Applications Menu shows all apps in the grid as before