Webassembly: Make virtual keyboard work in single-thread mode

The virtual keyboard uses a FolderListModel to dynamically populate the keyboard with layout files. This internally uses a thread to monitor the folder.

Since the system keyboard does not work with WebAssembly on mobile, it will be very important to be able to use the virtual keyboard.

Two options:
1. Make FolderListModel work in a single-threaded mode
2. Make the virtual keyboard work without monitoring the folder when there are no threads (e.g. by just checking contents at startup or running with static list of layouts)

It might also be necessary with some changes in the QPA plugin to get the virtual keyboard in there.