[Performance] Opening fonts dropdown janks the whole app for a while

[ProgramCrafter]

My system has 2700 fonts/font variants installed (or so Koharu's backend sees it), and opening the list of fonts freezes the whole frontend for 6-10 seconds.

Version: cutting edge (5b0a3c1).
Tested GUIs: Koharu's own WebView; visiting the frontend from another browser (Firefox).
OS: Ubuntu-based distribution of Linux, on Wayland.

[mayocream]

related: #267