Work in progress implementation for #833

This removes the inotify dependency and file watching completely, as well as several other classes we don't really need (AppDb, AppIconCache). It's both due to using Gio.DesktopAppInfo.get_all() now to load the apps, but also for just a lot of refactoring in general

• There's no caching at all now in our logic, but the C-code for the Gio.DesktopAppInfo.get_all() API does cache.
• Most of the logic is now in AppResultItem.py, which I consider to be as close as it gets to the "app object". I want to move the LaunchAppAction there as well, but I'm not sure if this class is used by any extensions.
• The code to fetch the apps and filter them is now run for every keystroke, which might be something we want to avoid, but it was easier to implement it like this as there is no event for when the window is shown in the right place. And the performance is snappy as ever.
• There's an incompatibility with the app stat db, so I changed the name of the file to app_stat_v3. Previously it used the desktop file path as the identifier, but now it uses the desktop app id (which is what it should have used before, to avoid duplicating apps)
• This will fix Custom desktop entries in home dir should persist if the global variant updates #320 as we're no longer handling files directly, and should also fix App scanner doesn't follow symlinks #310, assuming Gio.DesktopAppInfo.get_all() supports symlinks (if it doesn't we should ask users to submit an issue to the maintainers of Gnome/Glib).
• SortedList and SortedCollection were used for the file browse mode as well, but I removed this dependency in a separate commit before this, so they can be removed completely. We really don't need them. It's easier to sort normal python lists in just a few lines of code, and performance isn't an issue here.