Includes fixes from Christoph Haag:
```
st/gui: Fix segfault when starting monado-gui without arguments
```
```
st/gui: Run gui_prober_select only in the modules that need a prober
This also speeds up starting up the monado-gui main menu.
```
Co-authored-by: Moses Turner <moses@collabora.com>
Co-authored-by: Jakob Bornecrantz <jakob@collabora.com>
Co-authored-by: Christoph Haag <christoph.haag@collabora.com>